Arch wiki is still relevant, I still use it as a reference on my NixOS box.
a large chunk of your Linux knowledge no longer applies
Your pacman and pacur (or whatever the name of the air helper soup de jour is this week) will no longer apply. Most of my linux knowledge was still applicable. You have all the same programs that run in the same way as they do on arch.
you can’t compile from source
Sure you can. Want to compile everything from source? Just turn off using the cache. NixOS is a source based distro.
everything is different
Also no. I use the same programs I can get on most other distros.
the nix language kinda sucks until you “get” it, etc.
If you have ever used another functional language its fine.