Lisp is simple to compile. Even C (at the level this tool works anyway), also basic, and maybe other 100 languages (again, not the whole compiler, but what this tool shows).
replyCompilation per se is not difficult. Doing it with good error recovery, good feedback in case of error, and the optimization is difficult.
This tool does not tell me those parts (for that is gotbolt) this is about how the first layer of interpretation works, and not for making compilers, but for understanding what the code actually means, because is not obvious at first sight.
I agree with an overall thrust of “C++ is overcomplicated and has many moving parts and a lot of implicit stuff happening in the background that makes it difficult to understand”. But I do want to briefly note that Lisp environments have macro expansion steppers for a reason too.
replyClassical Lisp 1.5 yeah, modern Lisps not so much, hence why there are famous publications like Lisp In Small Pieces.
replyOnly if you mean K&R C or the early C89/90 compilers, before the extension explosion, newer language standards, and modern ISAs.