I'm not sure it's fair to derive that a majority vote against the PCs indicate that they would have lost under a different system like, say, runoff voting. I imagine many voters who voted Liberal or, yes, NDP, would put the PCs as second on their runoff vote, not the opposite "left" party.
Similarly for proportionate representation, we would end up on a constant minority parliament which would shift political platforms around and would likely see the Liberals supporting a PC government. With the PC's having the 413 as a strong part of their rather limited campaign platform, I would struggle to see the Liberals being able to move to cancel it.
Regardless, it's all speculation and the vote results we had this month are a reflection of the system in which we operate under. The results would most likely have been wildly different with a different electoral system and to speculate on that is exactly that, speculation. The PCs won more seats as a percentage of parliament since John Robarts in 1961. It's very hard to challenge their electoral mandate.
Again, considering polling has indicated strong majority support for the highway in the 905, I would be surprised if the PC mandate says the opposite.
While it's far from conclusive, looking at the two polls available on it, one showing majority opposition and one showing majority support, with the differentiation being one including the 416 and one only polling the 905, we can assume that opposition is concentrated in the Old City of Toronto where the Liberals and NDP did best. This opposition then presumably tapers out as you move away from downtown, and likely switches to majority support close to the City border.
Why the thought that majority opposition on a project in Peel and York Regions from people in Toronto should override the majority support in Peel and York, as well as the democratically elected at large government on the provincial level, I'm not sure.
I would bet there are a lot of issues that receive majority support from Downtown Toronto that would be wildly unpopular on the provincial level, and from what I have seen, this is just one more of them. Ontario outside of the borders of the City of Toronto is a very different place than within it, and there is a reason much of Ontario feels like Toronto too often forgets that.