I guess that they want you to create a master key and re-enter all accounts and their passwords, so they will be store encrypted. If you trawl around distribution, you may find them unencrypted somewhere.
Interesting, I recently set up replication on SQL 2022 CU18, and there is no master key in the distribution database. I did not notice any warnings, but then again I used the wizard. (I only set up replication for a lab test, to help with another forum question.)