Mearls is talking about the 2014 D&D 5e, because he can't be talking about the 2024 version, as he was laid off before that and was working on MtG for years at that point.
And while Mearls might be dismissive of the 'Holy Cows' in D&D and having to resort to 'hacks' to make them work, someone conveniently forgot that their work on 4e (which Mearls also worked on) did away with a lot of D&D 'Holy Cows' and it pretty much destroyed D&D as a brand for 7 years (no matter how mechanically sound the system was)... So yeah, 'Holy Cows' are important for the D&D brand, and more importantly for it's players.
That said, the Legendary and Lair stuff I always found problematic in D&D, 2024 made that a bit better from my perspective. But I still think that the Legendary Resistance stuff is too much of an automatic cancel button. On the other hand, so was the 2014 3rd level Counterspell (the 2024 version changed drastically).
Mearls often sounds like a grumpy retired veteran regarding D&D these days, that wasn't always the case though.
And while Legendary Resistance can be a cancel player action button, it does have advantages. It's fast, it expends player resources, and by itself it's also a resource. And as it only changes a failed save to a successful save, it either cancels effects or mitigates them (like damage). I think people underestimate how important fast combat is. We're noticing that fast combat is more enjoyable for everyone involved and 2024 improves on that dramatically by making ability descriptions more concise and streamlining abilities (multiple different attacks into one). There is no perfect RPG (especially not D&D), as such I prefer a faster flawed D&D then a slower slightly less flawed D&D.