Everyone can read the chat. Since you can read it even if you're not logged in, it would be incredibly silly to forbid it if you were logged in but low rep.
The 20 rep threshold is for actually chatting, i.e. writing messages.
FurthermoreHowever, chat room owners canshould be able to give users explicit permission to talk regardless of reputation or other settings.
Even though the description on this feature says exactly that, it does nothing unless you're a moderator on a private room.