You're right! I didn't know this word and assumed, wrongly, that it meant something like "deadbeat". I should have checked first; got caught up in the heat of the discussion.

However, that makes the reference to this incident even more incomprehensible. Who said anything about off-duty pilots?

Being a qualified pilot (or dressed as one) is not the point; the point, as has been stated many times now in this discussion, is that:

1. whoever is flying the plane today does not need to go through security, because they'll end up in the cockpit anyway

2. whatever piece of ID is enough to get someone at the commands of a commercial airliner today should also be enough to get past ordinary security procedures designed for passengers

That's the thing. Deadheading pilots are not off duty. A duty segment for a pilot can include, flying (as pilot) from LA to Dallas, deadheading to Chicago, flying to NY, then deadheading to Miami and finally flying back to LA. Not a very efficient use of the pilot's time, but still possible and they're paid by the airline for the whole thing and are on duty.