I'm not sure I'm getting it? is UASF another version of SegWit that could (and should) be activated right away without the need of miners signalling it?
Yes, and no. Obviously Bitcoin can't fork without miners in its current state, therefore at least *some* signalling is required. The more the better.
If yes, then wouldn't that be the exact definition of "centralization"?
No.
What it basically does is that it sets a flag day at which the miner/pool would produce Segwit blocks. This proposal does require wast support from the users & economy to work though. Generally it seems that this support already exists. All >= 0.13.1 Bitcoin Core nodes are compatible with it, and it should be compatible with any other existing Segwit implementations.
Notes:
- This proposal is in its early stages and is still being worked on / fixed.
- It is not within Bitcoin Core, but exists as a *patch* from an unknown third party on Github.