This is how currently we defining Low 2-C, because when one destroy universal space-time continuum, you destroy uncountable infinite amount of copy/snapshot of the universe, and from here you have 4D, due to uncountable infinity.
However, we can apply the same logic to smaller space-time dimension, it can also be Low 2-C, since no matter what the size of the pocket dimension, uncountable infinite amount of something 3D always result in 4D. And even if we not assume the "time" of said space-time dimension is finite, the result is still more than infinite amount of 3d which result in High 3-A. Since like Agnaa said, any non-zero length time result in uncountable infinite because in real number, you have infinite length between each number, applying it to temporal dimension, you have infinite amount of copy of 3d on the length of the time axis, even if it is just 1 second long
However if we using this logic, it can inflate the power of character that can destroy smaller than universe space-time dimension which why despite the contradiction, the wiki only accept space-time that is at least observable universal in size to be 4D while reject smaller size, or you know we could have High 3-A character even if they only destroy room-sized space-time.