I heard that from a staff member, who referred me to AKM, who referred me to DT, who didn't respond to me asking.
The reasoning is definitely incorrect, but I can't quite tell if it's a common standard or not. I
think I asked Ultima about a similar case around 2 and a half years ago, and I
think he felt "At least 3-A, possibly 2-C" was appropriate in cases like that (but I could be misremembering and have come up with that rating myself). If it is a standard, it's not widespread enough to hit the pages I made based on that evaluation, such as
God Above All.
To elaborate on why the reasoning is incorrect, such statements that don't mention space-time often don't actually establish the universes as being separate space-time continuums. There are many dirt-common ways that multiple "universes" can exist in the same space-time. There need be no dimensional barriers being breached.