Until WatchOS 5 the APIs to do something like Spotify didn't really exist. There were workarounds, like by abusing the workout API, but unsurprisingly these has significant drawbacks, were unstable and Apple cracked down when they found API abuses.
Apple "blocked" Spotify only in that it hadn't (yet) released APIs that supported their use cases.
It makes me wonder how disingenuous their other claims are.
I do think services should have more options than Apple allows for accepting payment.