No, you can just grab a generated client in a language of your choice if that language happens to be supported by protobuf.
This is a massive regression from the openness of HTTP REST APIs.
But it's trivially true in the case where the provider does so wish (as it would be true of any protocol with a fully-functional HTTP gateway infront of it), and it's untrue in the case where the provider doesn't.
And the provider has to do extra work:
"[offering an HTTP gateway] required adding custom options to gRPC definitions in protobufs, and add an additional container running this reverse-proxy server." ( https://wecode.wepay.com/posts/migrating-apis-from-rest-to-g... )
This still seems like a massive regression in openness to me.