I wanted to think that was the case, especially since it'd mean Time is Low 1-C, but I just can't agree.
Marc Platt uses the term Omniverse very liberally if you look into his works. For example, the Sixth Doctor describes the area of the Vortex he travels through as the omniverse.
It's almost exclusively presented as infinite possibilities and infinite universes, not multiverses.
Well, problem is, is that before, the Time Vortex WAS the gateway to all other universe (Ala Spiral Scratch), but they changed that later on and it only got worse:
Because in New Who, they changed it so that alternate Universes start after the Time Vortex, with each Time Vortex being its own Universe, shown when Mickey, Rose and the Doctor fall through the Vortex in their Universe to another Vortex which the TARDIS can't draw energy from.
Some of the Old Who books put the Alternate Universes after the 11th Plane, that other Universes can have any amount of "rooms" in the Hotel that is the Universe as long as the laws of the Universe are different.
Then there's all the stuff about the Chronovores being able to access and eat Dead or dying Timelines, so it makes you wonder.
For me, I think Alternate Universes starts after the Time Vortex but before the Chronovore Planes.
Also, kinda another reason why I think the 11 Dimensions in Doctor Who are planes, not Dimensions in terms of Axes, is because at the level of the Void, which would sit between the Chronovore level and the Time Vortex, Dimensions no long matter, the Doctor just flat out says, the Void is undimensioned:
DOCTOR: The space between dimensions. There's all sorts of realities around us, billions of parallel universes all stacked up against each other. The Void is the space in between, containing absolutely nothing. Imagine that. Nothing. No light, no dark, no up, no down, no life, no time. Without end. My people called it the Void. The Eternals call it the Howling. But some people call it Hell.
RAJESH: But someone built the sphere. What for? Why go there?
DOCTOR: To explore? To escape? You could sit inside that thing and eternity would pass you by. The Big Bang, end of the Universe, start of the next, wouldn't even touch the sides. You'd exist outside the whole of creation.
Also, I love that name "The Howling"
Alternatively, you could put the Howling outside of the 11 Planes all together, as the Doctor says it's just outside of all creation.