"This repo is a fork of the Go repo (https://github.com/golang/go). Nearly all of Biscuit's code is in biscuit/."

From one of the linked papers:

"Biscuit has nearly 28 thousand lines of Go, 1546 lines of assembler, and no C."

There is C code in biscuit/user/c, but it appears to be userland test programs, not kernel code.