Functions have a unique interface. It's always the same. Params in, return value out. Plus an imported function is something custom, by definition. You don't have to wonder.
Now you encounter custom syntax. It will not be clear it's custom first as you have no import path. Maybe it's a variant you didn't know about. Maybe it's part of this particular lisp flavor, after all there are so many.
You google it, but it's hard to google syntax so you are not quite sure that no result mean it's custom, impossible to search or just an obscure feature of lisp. After all there are many too.
So you try to decypher the syntax. But there is no contract for it like for a function. No unique entry point and output. It could be doing anything.
Now wihtout an import path you are on a fun hunt for its possible definition with grep.
When you are finally reaching it, it will be 50+ lines of dark magic you now have to decipher because syntax is hard and not trivial code makes awesome features.
In some lisp shops it works because you have three 10x programmers with years of lisp experience working in the same office. In that particular configuration, lisp is cataliser to this great potential, with xp and communication compensating the rest.
But the world is not solely revolving around this perfect combo.