It's an underground LRT, not a subway (as others have already pointed out.)
This was in fact part of "Transit City" - though I believe under "Transit City" only 5 km of the Eglinton LRT was to run underground and with this announcement, it will be underground all the way from Black Creek to Kennedy, where it would run surface and replace the existing Scarborough Rapid Transit system.
I haven't read anything about it running to the airport, though it could.
The GO Transit Georgetown line is currently being upgraded from one track to four tracks and it (the Georgetown line) is basically the CNR Weston Subdivision and it runs out to the airport and to Georgetown. GO is planning to build 2 tracks into the airport which will ferry commuters back and forth between the airport and Union Station.
The GO Georgetown line is immediately west of Black Creek Drive, right by the old Kodak factory (which has been recently torn down)
LOGICALLY, one could build a GO Station on the site of the old Kodak plant which would be the terminus of the Eglinton LRT and it would connect into the GO line to the airport. If you were coming from the airport, most likely you would prefer to go right to Union Station, however you would have the option of hopping on the LRT along Eglinton.
A great idea if I say so myself.
The good news is that design work had alread commenced on the Eglinton LRT under Transit City. The designs could now feature underground LRT from Black Creek to Eglinton. I am aware that at the time that the design for Eglinton LRT was being done under Transit City, there was considerable debate as to whether or not the tunnels for the LRT should be at least constructed to a subway standard for potential conversion in the future. (It is slightly cheaper to build the smaller LRT tunnels than subway compatable tunnels.
To me, it's a no-brainer. Build the tunnels so that you could convert them in the future to subway. (Take a lesson from the Bloor Viaduct which RC Harris had constructed to support a subway 40 years before Toronto even dreamed about a Bloor subway line.)