IIRC, someone linked a video showing that they are universe-sized.
If they aren't universal-sized, than whether or not they are a separate time-space is irrelevant. It's on the Tier System page (it used to be High 3-A but we got rid of that). I'm starting to think it may have been a mistake but that's beside the point. That means if they're only 3-B sized, whether or not they are separate time-spaces is irrelevant.
While Dormammu has multiple reasons for his 2-A rating, one of them is in fact sustaining the Dark Dimension, which is a metaphysical realm said to be made up of "worlds beyond measure". Not only is that not 2-A (it's 2-B at most) but it's in the same ballpark as this.
The Underworld in Saint Seiya is considered 2-C because each realm that's part of it is universal in size. The Hyperdimension for example is made up of billions of galaxies. Granted, each realm was also stated to be beyond space-time.
Basically the only thing that matters here is the realms' size, not whether or not they are temporally disconnected since that's hardly able to be proven under most circumstances. And Tier 2 isn't that hard to get.
On the opposite end, the default size of a universe is 3-A. That includes other realms considered to be universal in size. That's why the Night Dimension is currently considered likely 3-A and not likely Low 2-C. Considering the changes to the system, it's unlikely that multiple 3-A realms actually can be considered 2-C and above without further context like in Saint Seiya. Pritti is absolutely correct that the realms being stated as "beyond space time" helps significantly, as it tells us the realms are temporally disconnected from the ordinary universe. Again, evidence that they are themselves temporally disconnected doesn't need to be significant, it just needs to believable enough that they aren't all the exact same realm stretched for infinity (something has to separate them).
So in summary:
Universe-sized realms only need to be separated from the ordinary universe(s) to be considered their own universe. They do not need to be proven to be spatio-temporally disconnected from each other. If they do, we have a problem with our standards because many verses got by without this ridiculously strict requirement. Something simply needs to be separating the realms themselves. It can be a dimensional gate or a portal, as long they aren't literally part of the same realm stretched across infinite light-years.
However, on the flip side, the base size of a universe-sized realm is the size of the observable universe. So unless it is outside the time-space of the regular universe(s), it will not be considered enough to pass as Low 2-C equivocal (meaning you won't get 2-C and up). Remember that alternate universes are 2-C because it's a multitude of Low 2-C, not 3-A (this is mostly why Universe 7 is not 2-C sized). The only argument I care about is the size of the individual realms because that's the argument that affects our wiki standards.
