Yolo From what I've seen on the freeflarum website, it is! It's called Members Class. The CSS code should be something like this (without testing it, let me know):
First we hide for everyone as luceos said:
.UserPage .UserCard-info { display: none !important; }
Then overwrite with the class added by the extension to show it to logged-in users:
.logged-in .UserPage .UserCard-info {display: inherit !important;}