Rob Pike was definitely not a PL expert and I don’t think he would claim to be. You can read his often-posted critique of C++ here: https://commandcenter.blogspot.com/2012/06/less-is-exponenti...
In it, he seems to believe that the primary use of types in programming languages is to build hierarchies. He seems totally unfamiliar with ideas behind ML or haskell.
Rob Pike is not a PL theoretician, but that doesn't make him not an expert in creating programming languages.
Go was the third language he played a major part in creating (predecessors are Newsqueak and Limbo), and his pedigree before Google includes extensive experience on Unix at Bell Labs. He didn't create C but he worked directly with the people who did and he likely knows it in and out. So I stand by my "deep, not broad" observation.
Ken Thompson requires no introduction, though I don't think he was involved much beyond Go's internal development. Robert Griesemer is a little more obscure, but Go wasn't his first language either.
Re-reading Pike's C++ critique now, I do chuckle at things like;
> Did the C++ committee really believe that was wrong with C++ was that it didn't have enough features?
C++ was basically saved by C++11. When written in 2012 it may have been seen as a reasonable though contrarian reaction, but history does not agree. It's just wrong.
I'm very much NOT saying that every decision Go made was wrong. And often experts don't predict the future very well. But I do think that this gives further proof that while not a bad coder by any means, no Pike is very much not an expert in PL. Implementing ideas you had from the 1980s adding things you've learned since is not the same thing as learning lessons from the industry in that subfield.
I would actually say even in 2012 the observation was probably wrong already, but notably Pike actually made it in the mid-aughts and was simply relaying it in 2012. However, being wrong about what was good for the future of C++ isn't really here nor there; Thompson also famously disliked C++. Designing a language that intentionally isn't like C++ does not equate to me as a rejection of expertise.
My point of view is that Rob Pike is a brilliant engineer, but a little too much opinionated for my tastes.