Well, this is certainly new.
Hackers in GTA Online have been able to cause all kinds of mayhem with various paid mod menus. We say "hackers" simply because it is shorter than saying "immature children buying mod menus with their parents' credit card" each time we refer to them, but you can be certain that 99.9% of the people you see hacking in Online didn't actually code the hack themselves (not that doing so would make it okay...).
Ranging from the regular stuff like spawning cash, instantly killing everyone in the lobby, spawning limitless props to crash the session all the way to stealing player's cash through insurance fraud (which thankfully has been fixed), hackers have been reported doing pretty much everything in the game that legit players are prevented from for the sake of balance.
However, hackers unlocking DLC before the official release date is something never before seen. A player has reported on a GTA Online related Subreddit that they have encountered a Prototipo, one of the recently leaked but not yet released DLC cars, in a race.