As far as I know, Drops/VHS uses the game set by the streamer to determine what game is currently being played.
So yes, a streamer could set their game and title to Zombidle and then go play overwatch, leaving viewers to earn drops for Zombidle.
And yes, you could link Twitch to the login service for Zombidle to confirm the streamer is playing the game and has their Twitch set to the game, yes at that point they can leave Zombidle open and go play Overwatch.
Basically you just described some of the loop holes, but all Twitch provides to game devs is the ability to see who is watching a streamer who says they are playing your game. It’s up to you the game dev to confirm the streamer actually is playing the game rather than another game.