I'd call both of those mega-adventures (the kind that provides enough material for a whole campaign) rather than adventure paths. They're more akin to the older adventures like Night Below or Red Hand of Doom.
I think the serialized nature, usually with very distinct steps, is important for something called an adventure path. You also have a storyline developing over the course of the path - it's not just "Here's a big problem, and here are steps A, B, C, and D you need to do to stop it" but "Here's problem A, which you need to fix. OK, now here's problem B that builds upon A, so you now need to fix that too. And after that you notice problem C which you also have to fix." and so on.
Take Princes of the Apocalypse, for example. I'll put it in spoiler tags:
[sblock]The core plot is "Find the missing delegation", which quickly morphs into "Stop the elemental cultists." This goal takes you through what is essentially a 13-level dungeon (four Haunted Keeps, four Temples, the Fane of the Eye, and finally the four Nodes). You can basically do these in any order, even though you might then run into things that are more or less difficult than appropriate.[/sblock]
Basically, you have a single goal that's handled in multiple stages, but you can do the stages in any order and the whole thing is clear at a fairly early point.
But compare that with Curse of the Crimson Throne - see the link for short descriptions of the adventures. Each of these is a distinct adventure that builds upon what comes before. It's a path where each step leads to something new and building on what came before.
Hoard of the Dragons is, I think, the closest Wizards have done to a "classic" Adventure Path for 5e. The adventure has a number of distinct steps, each building on the one before (village attacked, find out the source of the attack, stop that band, learn there's more going on, follow up on that, travel to Waterdeep, then into the Mere of Dead Men, and so on).
