Why not? On-premise and autonomous/decentralized are two completely different aspects.

How so?

You can totally have an on-premise appliance that behaves like a black box, is remotely managed by the vendor and depends on their cloud services for even basic functionality. (Not saying that this is what WhatsApp's solution is – I've never looked into it!)

I wouldn't consider that to be on-prem at all.

That's called malware I think.