We don't know what would have been the outcome since this never happened, but he saw Unity as a valuable core technology, so is reasonable to assume (whether you like Zuck or not) that Meta would have probably invested some serious money in making Unity (the engine) better.
Now that we are past that, we are looking at a company that can't grow its game engine business anymore, so they have started to invest in app/game monetization. But likely at the expense of being less focused on improving the game engine.
So the logic here is that Unity under Zuck (who is ironically the king of ad revenue models) would likely have thrived as a game engine. Instead, Unity as an independent company is now seen as nothing else than adware leverage.
