Have you watched Leinster play over the last 2-3 years? Fitz has primarily played at 13 and the Canada game was the first time he was given a decent run out at 12 (since then he's been at 12 ofc). The last time he got a decent amount of game time at 12 was back in 08/09.
Fitz was insane at 13 (both defensively and offensively) and definitely should have started at 13 for Ireland over Payne. He won man of the match last year against Wasps in Rd 6 when at 13 as he was a freak in defense. If anything Fitz is out of position at 12 considering how well he's been at 13, but as usual, most rugby fans' memories are short lived and can't look at the bigger picture when assessing players, teams, strategies etc etc.
He was unbelievable against Ulster last year:
https://youtu.be/GcNnbXbAHJY?t=1h27m3s
There are so many highlights of his play at 13 (both in this game and throughout that season) but if you play from 1h 27 mins 3 seconds, you can see how devastating he can be at 13.