Currently the Weston area has 45 diesel trains running by every day and that figure includes GO, VIA, and CN. In just 3 short years that figure is going to soar to 206.
Most of the increase (assuming you've got the numbers correct) are the low-emission airport trains. These will take significant numbers of taxis, buses, and cars off the road, which have significantly greater emissions.
I know that there's a good case that providing mass transit, merely empties the road, which then fills up again with other users, and there's no real cut to traffic - but that rule only applies at peak. Most of the airport trains are off-peak, when there is already excess capacity on the roads, so there would be little additional cars, buses, and taxis replacing the ones that switch to the trains.
The net effect is that there will be less emissions.
This then, is clearly not about this project, which will significantly reduce emissions over the current situation. But merely a case of people whining about it being there back yard.
Remember how long this group whined about other issues other than electrification, before they realised they might get traffic with that issue? Some of their complaints were very NIMBYish, such that cars would have to drive an extra short block to get to the station, because they were only building 3 vehicle bridges near the station and not 4 (the 4th only being for pedestrians).