My understanding is that the TVA exists outside of "reality" itself, or is its own reality all together. Thereby not making it another "universe", but something completely different all-together. A window from one reality, to another (or others) if you will.
A way of explaining it is that the TVA don'tdoesn't exist in same same "space-time-continuum" as everyone else. They just pop in and out of it as necessary - trimming the nexus events of all realities as they deem necessary, in order to maintain the "proper flow of time".
In this way, I don't believe that the TVA is the "greatest power in the universe", rather it exists outside of the other realities - and does not follow the same rules. What is deemed as "powerful" in one reality, simply isn't in another, and vice versa.