I would argue even if it wasn't STATED, it is an implicit necessity of making a competently designed 5e. Balance cannot exist intact with it at the sorry state 5e has is in with the disparity, and as a result neither can meaningful Monster and Encounter Difficulty guidelines.
Do you feel it succeeded at addressing this issue?
So. You'd be hard pressed to find someone who hates 5e more than I do. Level Up put a LOT of work in and made an actually good system.
Skills being disjointed from set Ability Scores means Martials get to do more outside of like 3 Skills that are easily eclipsed by Spells.
Spells were actually brought in line with a mathematical backing that designers didn't break because "They're iconic" or because they just didn't design them well.
Martials also actually get...stuff! Combat Maneuvers are the obvious one and most comparable to spells, yeah. But they also get Social and Exploration abilities to actually exist.
More benefits from Short Rests means they're more worthwhile to bother to take instead of 'If you can take an hours rest safely you can usually take 8' benefiting Casters entirely.
Also the expected Adventuring Day scaling with levels, they just aren't designed around 'Can still function 8 fights in' which no one really does anymore.
There are some remaining issues I wish were different, absolutely. But those are all wrapped up in the Skeleton of the system, which has to remain the same because backwards compatibility is largely what makes the system financially viable. So I find it hard to really complain about those things that were out of their hand. I'd love Short Rests to be Short, and Feats to be more plentiful and not an ASI alternative for example, but the system Skeleton assumes those things and keeping those keeps Level Up as more of a direct replacement than an alternative.
(Copied from my Discord post about the same question earlier today in case that wasn't you)