There is almost certainly no such rule, FAA, TSA, airline, or otherwise. Ever notice how every time flight crews make up some random BS it is either a "new rule," or "since 9/11", or "for security?" It is almost certainly the FA abusing her authority to make her situation more "comfortable" either for socializing, galley activities, or paranoia.

IMO a verifiable case of a crew member making up a rule to intimidate a passenger should be immediate grounds for termination.

There have been various indications at times there may be some directive suggesting restrictions on passengers "congregating," but nobody has ever been able to document it to my knowledge. And no airline yet has implemented the necessary protocol to avoid congregating/lines: numbered tickets for bathroom lineup and aisle passes for walkers.

Next time a FA tells you to go sit down for a silly reason like this, tell her that your physician said you need to walk around to avoid DVT, and your lawyer advised you to write down the name and employee ID number of anyone who would not let you follow your physician's request.