I believe they are referring to the fact that most JWT-based auth systems use one-hour token expiry and have no ability to remotely revoke tokens. You can only revoke the user's ability to get the next token. This often leaves a one hour window between when you want the user locked out of your system and when they are practically logged out.
The only way I know of to implement instant revocation in a system like this is to keep a blocklist of users/tokens that is constantly checked, which can be slow and removes some of the benefits of JWTs in the first place (that they carry all the auth information you need).