Note that let over lambda is ... let's say controversial among lispers. I think it is very good at teaching how to write macros, but very bad at teaching what macros one ought to write. It's a bit like that bit about making ruby accept bare words in Wat[1]. The fact that lisp lets you do the things that LoL presents is fine, learning how to do this is good. If you actually do this in production code, that's bad.

1: https://www.destroyallsoftware.com/talks/wat (45 seconds in)