I agree with OP, but just remember, EVERYone has to work on how they port games to Stadia. Everything will lag behind a little, and what we can hope for is developers to reach parity and integrate Stadia ports with the rest of consoles. However, with holidays and time constraints, there's little wonder why developers prioritize Stadia lower on the support chain.
It isn't fun, so hopefully over the next few months as we close in on the Stadia Base launch, this becomes a nonissue.
To add a silver lining to this cloud using conspiracy theory, maybe they're delaying that update because they're close to having performance/graphics significantly improved and want to drop that enhancement at the same time.
You will not find the dlc in a store, its already installed into the online part.