Why would offering an API mean that the service is decentralised? It doesn't - it just lets you get some data in or out. You are still beholden to a single party that can cut you off at a moments notice.
> I'll also note that - in my opinion - Blockchains aren't really decentralised. There's no way you can do an "Off-Chain" transaction. Everything has to be recorded on a single ledger.
Not true at all. Bitcoin has a Layer 2 solution where transactions are done off-chain in a non-custodial manner. Those don't need to be recorded on the ledger, only the settlement is (if you decide you do not wish to participate any longer).