If you listen to what he has to say, it’s quite interesting. He would occasionally use one to step through an entire frame of gameplay to get an idea of performance and see if there were any redundancies.

I really tried but could not take to Lex Friedman's interview style.

I think you should substitute “code” for “domain” in the last paragraph.

John Carmack knows his domain very well. He knows what he expects to see. The debugger gives him insight into what “other” developers are doing without having to modify their code.

For Carmack, managing the code of others the debug environment is their safe space. For Kernighan et al in the role of progenitorous developer it is the code itself that is the safe space.

If you're doing cutting edge work, then by definition you're in an area you don't fully understand.