"Keeping public keys anonymous" means you never share publicly that you are somehow connected to a certain address. You can use one of your public keys in transactions with your counterparty, but outside observers won't be able to determine who the participants of the transaction are. Undergoing KYC on a centralized exchange is the opposite of keeping your public key anonymous because the exchange will know that particular public keys belong to a particular identity. But you can't use aexchange without revealing the information about yourself and your transactions! When you combine multiple public keys in a single transaction you also reveal the fact that you control them all; it's called a "common ownership heuristic." CoinJoin tries to break this particular heuristic by combining inputs from different users and creating outputs of equal amounts.