Why would this be the case? Games preloading has been a thing for many years. This isnt like an MMO launch that there is additional impact when the game is being played.

Sure GTA6 may be a slight uptick in the amount of data being downloaded like any new game being released, but I would wager that the amount of data they are normally pushing out for downloads just on a regular basis (wether new purchases, redownloading games, game pass, etc) is already quite high.