But if the game is complicated (unlike dice), the chance is it's only partially on-chain.
This
game is running completely serverless and decentralized on the blockchain and it's quite complicated
With a customized blockchain made for gaming like Xaya has, this is doable and was already proven back in 2014 with
Huntercoin by the same guys.
Don't blockchain games still use centralized servers to run the actual game code itself?
They do, yes. Thats why technically they are no blockchain games, but just games with blockchain elements implemented and using buzz words. The devs are still able to do all sorts of shady stuff and dilute your assets.
For a game to truly be a blockchain game and decentralized then the game source code itself (not just the game data) would have to be stored on the blockchain, open source, and unalterable so that such centralized actions cannot occur and anyone can host it.
So is it already decentralized servers?
Xaya is exactly doing this and they are the only ones afaik. They have one game in beta now (Taurion is between alpha and beta rn) and everyone is free to check it out and play:
https://soccermanagerelite.com/. Like all games they build, SME is trustless, permissionless, censorship resistant, transparent, provably fair, autonomous, unstoppable etc. And play-to-earn of course