Nick, it is not unusual and the explanation lies in the nuance.
Captain is a rank, as opposed to pilot-in command or commander. So it is fine for two captains to operate together provide one is checked for the right hand seat. This latter requirement is nearly always the case for operating training captains, but it is also the case for any captain who has completed a small number of additional items on their recurrent training and checks.
Either captain may be designated as the commander, for the flight or series of flights, and to that end it doesn't matter which seat the relevant commander is operating from provided they are operating from either the left or right seat (as opposed to a jump seat.)
It may well have been a line check, although normally (not always,) line checks are conducted on two operating crew members being observed by a trainer on the jump seat.
If an airline is short of first officers, it will sometimes utilize a training captain, or other right hand seat checked captain to operate in that seat.
In order to perhaps avoid confusion, a captain operating as the first officer, might elect to refer to themselves as the first officer, but it is certainly not necessary, nor is it erroneous.