You are of course correct but they can't calculate an infinite number of journey options. I presume the idea was you spend 45 minutes pre flight and 15 post landing absolute minimum when flying. 2.5 hours by TGV is about 400 kms so say an hour in the plane including taxying etc. So that's an absolute minimum of 2 hours by air and likely more like 2.5 hours - so just set the limit at 2.5 hours by train
You'll note the Citizens' Convention on Climate suggested 4 hours - which is what I'd think is closer to reality for any centre to centre trip.
If they do it I could see it spreading across Europe