I'd like to point out that the example being bandied about isn't an apples to apples comparison. Invisibility does not grant stealth ability. It makes you invisible. Stealth is not being NOTICED. Not "not being seen". Invisibility does not affect the alertness of guards who should be both looking and listening for anything unusual around them. So perhaps in addition to giving you 100% concealment (or whatever it's called in DDN), it might give advantage to Stealth rolls. The invisible armored knight is still going to clang and bang as he walks invisibly past the guards.
Additionally, the level of the spell vs the level at which the ability is gained should have some play in prioritization. To say that a 9th level spell can't outshine a 1st level class ability seems silly.
The thing is, there's a difference between "Stealth" the design space (not being noticed, hiding, sneaking, ambushing, etc) and "Stealth" the skill. The L&L article is talking about design space over granular concepts. Think about all the ways that the Stealth design space can be addressed in the rules - off the top of my head, there are:
* How well you can hide
* When you can hide
* How you can be concealed
* What you can do if you aren't seen
The Stealth skill only deals with the first option, while the L&L article is talking about balancing the way all the different choices fit together.
So for instance, Halflings can hide when they're behind someone larger than they are. That makes them better at Stealth. Wood elves can hide if they're lightly obscured by natural stuff. Neither of these abilities are a bonus to the Stealth skill, but they're both balancable things in the Stealth design space.
The Rogue class gives you the ability to be proficient in the Stealth skill, to get a +5 bonus to Stealth at level 1 (via your choice in Expertise). It gives you things you can do with it (synergy with Sneak Attack) and the Cunning Action, which means you can hide more often. Both of the rogue subclasses give you abilities in the Stealth design space, although neither gives a flat bonus to the skill.
The Invisibility spell means you're always concealed. *If* you prepared it *and* you cast it in time *and* you don't move after casting it, it means you can be Stealthy without even making a Stealth skill check at all. If you move, you'll still need a check but unless things are quiet it will probably be an easy one. It's a good effect, but a lot less than the Stealth benefits a rogue gets at level 1.
Now, if the spellcaster takes a background with proficiency in the Stealth skill, that'll help. Adding the Mobile feat might help give you things to take advantage of being Stealthy (getting in and out of combat to hide again, for instance). The Hunter's Veil spell would be useful, too, along with Silence. Add all of that and you are probably better at being Stealthy than a level 1 Rogue.. Although you still don't have things like Sneak attack, and you've spent a lot more than a single class level to get it.
(And I managed to get through all that without saying anything about Not Being Seen...
https://www.youtube.com/watch?v=ltmMJntSfQI )