Yeah, just read this but half of what was said isn't supported by the quotes and scans.
Can you tell me which parts are not supported by the scans and quotes, I'll clarify them.
Being more “vast” than the universes(which I didn't see a direct reference for) is implying some sort of larger plane that's not exactly ontologically superior to the baseline existence.
The thing is, BDE page tells us if a void of nothingness or any Conceptual domain is shown superior to physical reality (like dwarfing them, as Ultima explained in the quote) should be given 1-A rating, the lore treats the void as something totally beyond the comprehension of the lower reality, the multiverse. We get ishtar saying "our little world is but a small part of the vast world of the void"
I've also given a explanation as to why the void dwarfs the entire multiverse
The void children (a.k.a the ancients) can leave their universes and step into the void / emptiness beyond it, only they can survive it's existence erasure, if any regular character were to enter it, they would instantly get erased. The void children can go anywhere in the multiverse as
worlds and times are open to them, they can go to the past and future + anywhere in the 2-A multiverse.
But even these characters find it impossible to travel from one side to the other in the void without the infinity beacon, just because of how vast the void is. The 2-A multiverse itself would be a small object encompassed in such a backdrop.
The letter reads "the void is very large, and getting from one part of it to another is impossible", what makes this letter so Important is that it was written by a character who can go anywhere in the multiverse + to any time period, nowhere in the multiverse is unreachable for them. But it all pales in comparison to the void as she cannot travel through it, just because of how big it is.
And, from what ultima has repeated in all of his explanations, he needs an atemporal + aspatial entity who is also shown to be
bigger /
vaster than dimensioned things to be given Beyond dimensional existence (type 2). I've replaced the quote with a different one in the OP, check it out.
The FAQ page quote mentions "their 'otherness' is identical to their transcendence", in this case, the void is something completely different compared to the multiverse, it is a void of nothingness which lacks anything physical, while also being treated as something completely beyond the physical reality.
They are fundamentally different from the nature of the lower reality, and this different nature is precisely the source of their superiority over it. Since their "otherness" is identical to their transcendence, no expansion or extension of the lower reality and anything in it can possibly attain to them, as long as it maintains its particular nature. Put it simply: They are as powerful as they are alien.
This quote actually perfectly describes the void.