Per multiple sources, including the team currently maintaining the Protobuf toolchain within Google, proto3 was largely designed by Rob Pike. Of course the Protobuf wire format is quite a bit older, but some aspects of proto3 and Go's shared semantics (like implicit zero values) do seem to have come from the same mind.
That's a much smaller claim, proto3 schemas are a fairly minor evolution over proto2 schemas, mostly removing features. My impression was always that it was either removing things that were expensive to support in Javascript, impossible to serialize to idiomatic JSON, or that the team thought were misfeatures that nobody should ever use. That's a far cry from the original claim that Go and protobufs were co-designed "in the same meetings".
But even that limited claim is kind of hard to believe. Can you link to one of those multiple sources making that claim?
proto1 was designed by Sanjay and Jeff, proto2 by Kenton Varda (who later designed Cap'n'Proto), proto3 I can't remember who but I never heard Rob Pike being credited with this. He did write the very first Go proto binding package though and a separate serialization package called gob with a totally separate rpc library.
