Just finished Witcher 3 so I wanted some more Gwent. Umm... wtf happened? Am I missing something? Why is this version so much different?

Also does Thronebreaker have Gwent? What is that version like?

Thronebreaker also uses the new Gwent. The only place to play old Gwent is in the witcher 3, or with the physical card game

What were they thinking?

;)

Whether that statement is true or not depends on what level of detail one interprets the statement to be alleging.

For example, a strong argument could be made that the Thronebreaker Gwent is an entirely different game from the multiplayer Gwent, as the cards in Thronebreaker are way better and more powerful/souped up & more interesting than are the cards with identical names that appear in the Gwent multiplayer game.

Although certainly neither of them are the same as Witcher 3's Gwent.

As for your question, OP: the core of Thronebreaker is Gwent; it just has some running around a map and voice-over narration tacked onto it, as window dressing.

As for why is multiplayer Gwent so different from Witcher 3 Gwent, IMO it's because that kind of thing is what happens whenever devs think they constantly need to change a game just for the sake of change, based on the faulty idea that making changes all the time keeps the game interesting.