Earlier quoted context omitted.
Writing an OS in Lisp basically means adapting or developing, and then growing, a Lisp implementation on the bare hardware. Someone would only do this if they are simultaneously interested in OS research R&D and Lisp implementation R&D. This is different from someone working on an OS in, say, C. They just pick some C compiler and use it, without becoming a C researcher and implementor. (They might bemoan some things…
I'm not sure I agree with this. How much assembly would you actually have to write to bootstrap a Lisp? If I understand correctly, not much. And you could cross-compile from some other platform for as long as needed as you use the Lisp to write the OS.
You don't magically get SBCL's reader, garbage collector, hash table implementation, object system ... or anything else you might want in the OS.
(You could fork that system though to borrow those things instead of doing it from scratch.)