Plan 9's userspace was originally written mostly in Alef, a predecessor of Go, then translated to C. I wonder if the original version of the C toolchain was in Alef, too.
Unfortunately I don't think there is a easy way to check.
Not true. Plan 9's user space was written in C. Alef came later and was used for only a few programs. The bulk were still in C, including the compiler toolchain. The Alef compiler was written in C too, never in Alef.
In the second edition Plan 9 release, the last one with Alef, these programs were written in Alef: acme, aux/consolefs, aux/depend, httpd, md5sum, page (document viewer), postscript/tcpostio (driver for lp), and ppp.
I think he was suggesting that you wrote the compiler to use only the parts of C that could be converted to go, not that C was written to be convertible to go.
The more likely explanation is that the same people wrote the compiler in C and designed Go. The subset of C they found most useful is the one they used in the compiler and the same one whose semantics ended up in Go.