The joke is “Jim” sounds like “gym”, so he can say he goes to the “gym” first thing in the morning.

Yes, it is.

Edit: downvoted because I was able to sign up for a service recently?

Have you tried Firefox Relay? They have a paid tier that gives you a phone number and unlimited email masks. Yes, you have to provide a number, and yes you have to pay, but you wouldn’t be giving Meta or Discord your contact info.