See now this is interesting. I don’t think that this is something beyond the ability of PbtA games in a general sense. Certainly the GM is meant to think off things off screen and to introduce complications such as this. But how and why the GM does so is different.
In your example, this is based on a previous campaign. That’s something if not unique to at least far more common or likely with trad play.
I would think that this might be interesting if you have players in the new campaign that were also in the old one. Something I do in my 5E GMing has been to bring in these kinds of easter eggs from past campaigns of ours. It adds something to play, I’d say.
I think the bigger difference, here, is that PbtA does not consider
between-campaign things as part of its structure. That is, if you're going to do this, it would generally be part of the premise of a new campaign, rather than something unexpectedly sprung. For example, previous game comes to a close, GM says, "I think we all had a lot of fun in this overall world. How do you guys feel about our next game happening in <nearby region>?" And the players say sure--we heard interesting stuff about what was going on out there. They figure out what they want to play, and work with the GM to use those things to develop details about the region that weren't known before (since I doubt even the most ardent of worldbuilder GMs is doing things like laying out the full dynastic politics or economic rivalries or midlevel ministerial bureaucracy for a place no one in the previous campaign's party was from and none had visited at any point during that campaign.)
Then, at that point, the GM sets about developing Fronts, which (naturally) will be at least partially unknown to the players at the start. Some of these Fronts will almost surely
connect to the PCs, because that's one of the classic ways to get a party together--give them common enemies--but the exact nature and interests of these Fronts will become apparent as the constituent entities (people, organizations, powerful creatures, natural disasters, etc.) unfurl and develop. A
perfectly acceptable Front is an invasion from an outside source, especially if it's already known amongst the players (hence "no-myth") that this region is dealing with political instability and social unrest. Perhaps the Blue Turban Uprising (a group of violent rebels with legitimate grievances but not particularly legitimate tactics) is secretly--even to its own members!--funded by the <imperialist-colonialist country> military as a means to soften up the region for invasion. Perhaps diplomatic overtures from said country are secretly espionage actions in disguise, designed to reveal military strengths and weaknesses, undermine industrial and agricultural capacity, and acquire local assets that can provide intelligence or turn coat once the invasion begins. Etc.
More or less, you can totally have an invasion! But having it be a total blindside surprise--no "grim portents," as DW puts it, and no ability for the players to respond until it is Already Here--isn't really copacetic. Either the invasion should unfold naturally as a sequence of events that the players can
try to influence ("play to find out what happens"), or it should be baked into the "pitch" of the game as a thing that HAS happened, making it an explicit part of the premise that that is true and the characters will need to respond to it.
Dungeon World leads to plenty of surprises. I would know. More than half of the "plot" of the game I run is stuff I would never have considered myself--only occurring because the players declared actions and I responded. I, for example, would
never have predicted that the Bard (originally conceived as the typical "wine, women/men, and song" type) would not only become a double cambion (effectively, Because Magic, half-devil, half-demon, half-human) in two acts of profound self-sacrifice, but to further embark on a quest to
accept his role as the prophesied messiah-figure of an
assassin-cult in order to reform them from within and make them into an organization he could be proud of. And that's just the story of that one character! We've had five other similarly dramatic stories move in directions I never considered.
I just about took out a level 19 party (6 PCs, 1 sidekick) with 3 CR 17 monsters in the first encounter after a long rest. I had a decent chance of killing at least a couple, but they were unintelligent beasts so the tactics weren't exactly spectacular. They were still under the effects of a heroes feast and there's a paladin in the group. So ... how exactly is it that you can't challenge a party? This wasn't even a deadly encounter by my calculations.
The point of Tucker's Kobolds is that they are, quite literally, Level One, from the days when "level" was
literally synonymous with the
level of the dungeon you were on--so "20th level" enemies were those you would expect to find having delved down to the "20th floor" of the dungeon. They have 1-4 HP.
Tactics can do an awful lot. They can't do
everything. And they're not really going to turn "a tribe of maybe 100 kobolds" into something equivalent to CR friggin' 17. Just one
meteor swarm from the party Wizard is enough to guaranteed slay all of them. Even in the very unlikely event that 100% of the kobolds make their save,
and the Wizard rolls the absolute lowest damage possible, that's still 10 bludgeoning and 10 fire damage. They'd have to have
more than just resistance to
both damage types in order to not instantly die. And, sure, that's a 9th level spell that the Wizard might not have--but it illustrates how a 20th level party could quite easily stomp these kobolds simply because they have so little HP. Holing up in the warren isn't effective when the enemy can magically torch the place and just
leave.
I'm not saying you CANNOT challenge a party. I'm saying that,
very specifically Tucker's kobolds, would not present a challenge to a 20th level 5e party. Because high-level spells are just that powerful. (Edit: And they don't even have to be THAT high-level.
Sunbeam would be wonderful against a kobold warren. Only 6th level, and it creates sunlight around the caster, which gives the kobolds disadvantage, negating their Pack Tactics feature and
pretty much guaranteed killing every kobold in the repeatable line AoE--it does ~13 damage even on a successful save, CR 1/8 kobolds have 5 HP.)