Is Docker a hosting platform?

No, but it probably wants to be. Docker found it's way as a runtime for packaging developers software, and became the defacto standard for building and shipping software (web apps get the most attention, but containers are *not limmited to web apps at all).

How is Docker a hosting platform then?

You could/can use docker to host:

• Containers - aka "docker hub", and this is what Docker is most famous for, but costs them a *lot* of money to run
• Docker Desktop - Which now have more of a freemium model, costs money depending on the size of the company
• Newer 'BETA' platforms Docker is building around the protocol WASM

So in summary, Docker is a collection of opensource services resembling a platform, but perhaps still finding its way to long-term financial sucess and stability (which is true of many open source endeavours).