Many Google APIs were created before this guide. New APIs published at https://github.com/googleapis follow this guide. Having the same API available via both REST and gRPC is very valuable, as gRPC often provides 10x performance.
You might have been hitting the old JSON-RPC endpoint of that API, which has to return 200 to follow the standard. Or a gRPC version if there's one already.
Otherwise HTTP status codes for errors are used, and standardized internally.
I don't recall that specific scenarios, but over the past year I learned this with the maps API. As long as you're hitting a real endpoint, then it will return 200 - even when there are errors and it should clearly return a matching http status code.
I suppose it so the same API could be implemented in any given protocol, however, I don't think that is useful in many cases.