You don't have to pretend to be a pilot. Any cabin crew is allowed in the cockpit, AFAIK

Not just cabin crew, a lot of the time anyone flying standby is offered the jumpseat if there are no other seats available out of courtesy. Especially if they are an airline employee, but often non-employees too.

In US, and I imagine elsewhere....this is completely untrue for any commercial flights or honestly anything carrying legitimate insurance.

I have never heard of a plain clothes non-employee in cockpict jumpseat.

I don't think just anyone is allowed to sit in the jumpseat in the cockpit.

My dad was an airline pilot. Policy was you had to be in uniform to sit in the jump seat, and, yeah, it's not open to just anybody. If he was flying standby to get home, he would take it if no other option was open.

The 9/11 hijackers were trained as pilots though.

This is like a person who took a few python courses pretending to be a software engineer with a full time job, the lie becomes very clear after a few sentences.