GTAA is an independent entity, their plans are not related in any way to the Ford government. The transit hub was always a long term project, they simply put the idea on the table so that when government officials are planning transit routes they plan them with this central hub in mind.
A sliver is in the City of Toronto too. Also, I don't think it qualifies as federal anymore either, although I think the land itself may still be.Pearson Airport is federal. It is in Mississauga, Ontario, so has also municipal and provincial input, only because of the airport's location.
I wonder if you gave citizens a map of the GTAH and asked them to put pins whether they think transit should converge other than Union, how many pins would land on Pearson. Many, sure, but I'm not sure they even would get a plurality. This discussion is driven purely by the cash that GTAA has access to in the form of passenger fees which allows them to pass cheques to consultants in return for glossy PDFs and branding. And if having a transit hub suddenly didn't suit Pearson because of personal vehicle automation giving higher throughput via 409 etc., those glossy PDFs could quietly disappear because GTAA has no legislative mandate to operate a transit hub, and as a federal entity cannot be obligated to cooperate with Metrolinx/MTO other than when in their own interest.