He also said he worked on the alpha version of Linux.

As per[1], "According to a 2009 interview, Thompson now uses a Linux-based operating system." He probably uses multiple systems, including macOS in some capacity, am sure, but it looks like he uses/used Linux.

[1] - https://en.wikipedia.org/wiki/Ken_Thompson

I can't find the source right now but Russ Cox posted to some newsgroup or mailing list that most golang developers use OS X these days (rsc, rob and ken for sure).