I listened to archived LiveATC recordings & the crux of the matter seemed to be that Rwy 29 (The main runway) was not cleared & available so the only option was Rwy 34. Rwy 34 is 7000ft IIRC whereas 29 is 8000ft.

The pilot was a little unhappy on ATC (& this is born out by various related tweets) because they'd delayed departure from YYZ based on a NOTAM that 29 would be open from 0100 on the 16th. They arrived at YYT, were given 34 & held for an hour or more with a back and forward conversation with the airport about whether 29 was plowed, was it going to be plowed & why the NOTAM seemed to be inaccurate. Plus then a back and forward on whether 34 was long enough - the onboard calculation (pilot referenced an Ipad app) was too close for comfort so they elected to return to origin.

I'd trust the pilot's judgement on whether a runway was safe to land on or not, so I guess Air Canada's beef will be with the airport authority over the NOTAM.