Stubby is based on protobuf; where by "based" I mean layered on, i.e. protobuf is the encoding used by stubby to encode requests and responses.

gRPC is a reimplementation of Stubby suitable to be used outside of Google.

Yes, I think you are just using a different sense of "based on" than I am. gRPC is based on Stubby in the sense that it is influenced by the design of Stubby and uses the knowledge learned from creating Stubby.