TGV will take you into the city or to the airport (but not both) - depending on which actual train you are riding on. Conversely (in the other sens), you may catch a TGV at Part Dieu or you may catch one at the airport, but you may not catch the same actual train in both places to my knowledge.
Which is another way of saying that they are not on the same track - I guess.
on edit: this would appear to be the same sort of diversion that one encounters in French websites. A friend to many of us had the unfortunate experience of arriving (via TGV and bus)in Bourg-St-Maurice instead of Part Dieu, while his ride (me) was at Part Dieu waiting for him. Although it was January, he was carrying empty styrofoam shippers, not skis. Quel dommage!