tabards are not giving experience to factions in dungeons
Not all factions could be championed with a tabard. That was mostly a Wrath of the Lich King, Cataclysm, and Mists of Pandaria thing. (Plus capital cities, if you need their faction rep for something.)
Here’s the WowHead filtered list of tabards that can give reputation in dungeons. Which faction and what expansion are in the tooltip of each tabard.
none of these people are working
MoP didn’t have the rep tabards. It was one of the criticisms MoP had because you had to do the dailies instead. And you still had the 25 daily limit.