I'd be willing to wager that C programmers would be more comfortable working with a Golang codebase than Golang programmers would be working with a C codebase.

There may be more "C programmers" by number but a Golang codebase is going to be more accessible to a wider pool of applicants.

In my experience it takes a few days for a moderate programmer to come up to speed on Go, whereas it takes several months for C. You need to hire C programmers for a C position, you can hire any programmers for a Go position.

If they don't already know C though, how well will they cope with manual memory management?

How do people learn C without knowing about manual memory management? They learn about it as they learn the language. This can be done in any language that allows for manual memory management (and most have much better safeguards and documentation than C, which has a million ways to shoot yourself in the foot)

It will be a learning curve, but a much, much smaller one than learning C.

But the entire point of this line of questioning is that there are more programmers who already know C.

He's wrong: https://www.tiobe.com/tiobe-index/