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.