[grandalf]

> (If you are tunneling another application-layer protocol over HTTP, your argument makes sense, but that's not the general case with APIs.)

I'd argue that that is actually what most APIs are doing that are not purely "REST" operating naively on resources.

As we move up the protocol stack, one is the transport for the next. HTTP is not really the application layer protocol, it's a transport for a protocol defined by the API, at least for any API nontrivial enough to benefit from its own specific error/response codes.