I had this massive paragraph typed up but whatever.
Here.
Imaginary Space exists in infinite amounts OUTSIDE of regular four dimensional space. As said in the second paragraph. This means it is NOT a part of the universe. Hell, it even proves Imaginary Space, the SoQ and the Imaginary Tree are higher dimensional structures by virtue of being unordered. The Tree itself is still dangerous to be on, as Durandal and Otto show.
Real Space is 'the only space able to produce any sort of meaning' and that the 'constant relationship(s)' between objects only exists in a 'single space' which is 'also the four dimensional space around us (the only space of real numbers)'
This proves base reality in Hi3 and HSR is 4D.
The way the tree ACTUALLY works:
The Tree holds the worlds in its leaves. Accepted. Simple.
Between the leaves held on the Infinite Tree is Imaginary Space, between the branches, where 4D reality is at its most unstable (just like in Nagazora, curtesy of the Third Eruption). This is because the Tree holds all worlds and is the most stable because 4D base reality can only exist in leaves on the Tree.
This directly parallels the Imaginary Intertidal Zones in both Hi3 and HSR. Imaginary Space and Imaginary Intertidal Zones are repeatedly described as chaotic.
In Nagazora, both the Sea (quantum shadows are leaking in) and Imaginary Space (explicitly stated) is leaking into 4D reality. As stated if not sealed the world would've been destroyed.
Durandal also visits Imaginary Space to reach the Imaginary Tree itself to connect her bubble world to it in Hi3.
The Imaginary Tree and Imaginary Space's properties form the 4D reality of HSR and the various anomalies we see like the Imaginary Intertidal Zones. The Imaginary Intertidal Zones are where reality is weakest and Imaginary Energy is leaking in, like in Nagazora. Hence why it's chaotic and you need extenuating factors to travel through it. (Dudu's bubble world, Emanator status, special travel means).
Hell the scan above even refers to the physical plane

It's obvious the tree, sea and img space aren't part of the physical plane aka HSR's universe.