Like, nothing in Game Sonic's lore hints to the mechanism of how or why Super Forms increase the Durability greater than their AP. It should be an equal increase in both stats. Game Bosses who use Chaos Emeralds, or similar powers to the emeralds, to increase their stats, are physically harmed and sometimes killed, by Super Sonic.
The thing that indicates that their durability is increased greater than their AP is that they're stated to be invulnerable, and to tank attacks from those who are comparable to them in strength.
is that Super Forms are said to have invulnerability. Game Bosses who use the same power aren't said to have invulnerability, thus, are harmed(unlike Super Sonic) and defeated. But why?
That distinction exists regardless. Either they have invulnerability, or they have higher durability. Why is the former acceptable to you, but not the latter?
Maybe so, but the lowest possible interpretation needs to exist first. Just "gain higher durability than AP" doesn't exist as a lowest possible interpretation if non-Super Forms use stat increases from the same source but don't have higher durability than their own AP. The only possible interpretation is that Super Forms tap into an ability from the power source that non-Super Forms can't obtain.
Why doesn't gaining higher durability exist? Why can't they tap into an ability of the power source to get higher dura? Why does it have to be tapping into an ability of the power source to become literally invulnerable?
Ok, getting somewhere. What attacks can Super Forms not negate on their existential? Does 2-A exist in Sonic lore? Didn't Solaris consume the entire cosmology and Time and still couldn't hurt Super Forms?
This is not getting somewhere, this is just kicking the can down the road. After this you'd need to prove that Super Forms can exist attacks above baseline 2-A, then attacks two layers above baseline 2-A, and so on ad infinitum.
At one place you'd have to run out, but true Invulnerability wouldn't (well, not until the next existential level is reached, at least).