Interesting. Although I think the author advocates for going a step further than CCL. My understanding is that the GC of CCL is implemented by its lisp kernel [0]. The lisp kernel is a platform specific C / ASM program which seems to provide a runtime for CCL. The author states that the GC should be written in the
language itself.
It's not something I'm familiar with but I saw a similar question [1] on r/lisp a few days ago. u/digikar mentioned the Teddy library [2] which itself refers to numcl [3] in its readme. Both are available on Quicklisp.
[0] https://github.com/Clozure/ccl/blob/master/lisp-kernel/gc-co...