Neither of what you said is true unfortunately. Positional Advantage has nothing to do with weapons that have the Light property even in the *literally never used anywhere ever* optional rule of "Facing" in the DMG.
And if you think it makes sense, what is your response to the 56+ class abilities and spells that are completely invalidated by the two homebrewed systems?
The rule exists page 252 black on white in print. You can't blame them for making use of it and limiting it a tad.
As for second point - citation needed. "Invalidating" is a BIG word there, I'd imagine the actual truth is somewhat not there. Overall the point is that there will be plenty of changes and in my opinion many of these are deliberate changes to make the game less frustrating for people who do not necessarily get off having RNG miss streaks. I'm down with them tampering with AC values and adding a few extra sources of Advantage for that, it won't detract from my enjoyment of BG3 and likely will only increase it by reducing wasted actions.
If there will be spells that will be legitimately useless given these changes, then they either will be just that or will be amended (like Sleep, for example, btw). It's not unlike BG/BG2 - you also had a ton of useless spells there and these games were far from faithful 2e recreation - somehow the world did not end there, eh?
Larian doesn't limit Facing. They expand it, making it much more easy to use. And we certainly can blame Larian for using an incomplete version of an optional rule. Just as you can defend their choice. As far as I can tell there is no DMG component of the rule that restricts it to light weapons. I will be much happier if/when this is implemented, though I'd be even happier if it was limited to the rogue and/or changed to a flat bonus.
And yes, they don't invalidate 56+ class abilities. But height/backstab (dis)advantage DO make many of these 56 abilities less useful (e.g., why Barbarian reckless attack when you can just backstab? Why faerie fire when you can just climb a ladder or backstab?). Again, replacing height/backstab (dis)advantage with numerical modifiers would completely fix these issues.
Also: Larian's fix to sleep doesn't fully correct for the HP increase. >50% HP increase for most goblins but ~30% increase in HP that Sleep affects. Sleep goes from affecting 2-5 goblins to affecting 2 at max.