This is extracted from the MMEL on the TCCA website (it's the FAA MMEL, though)
8. APU Fire Detection System
1) Single and Dual Loop
C - 0 (O) Except for ER operations, may be inoperative provided APU is not used.
2) APU DET INOP Light
C 1 0 (O) May be inoperative extinguished provided:
a) APU fire detection system operates normally,
and
b) A fire warning test is performed before each APU start.
3) Dual Loop
C 2 1 (O) Except for ER operations beyond 120 minutes, one loop (A or B) may be inoperative.
4) External Warning Horn/Warning Light
C 1 0 May be inoperative for ground operation provided flight deck APU Overheat/Fire Protection Panel is continuously monitored.
So there are circumstances where the APU FDS is required. Doesn't sound like yours was an ER operation, but if for whatever reason they were going to need the APU then it's required. And if it was item #4 it's a no-go for flight.
Also the operator MEL may be more restrictive than the MMEL.