I'll be the first to agree with you that most 'minimal' lisp implementations are going to be missing important features (efficiency, memory management, ..), but

I'll be the first to agree with you that most 'minimal' lisp implementations are going to be missing important features (efficiency, memory management, ..), but is the parsing of s-exprs comparatively a difficult task? At just a glance, it appears that the library is slurping the expression and avoiding recursion instead optimizing for speed, memory usage, and error reporting, but it seems like you could implement a recursive descent parser if you weren't as concerned with those. If the author took their same approach to parsing s-exprs for a grammar like C, I'm not sure how successful they would be. I haven't really looked at grammars / compilers in a long time, but I'm curious if I'm off-base in thinking s-exprs are relatively easy to parse. Some grammars like brainfuck or assembly would obviously be easier.