The 3rd game was clearly different from the first two encounters. And they needed Embiid to win that one.
The third game was different because Serbia got hot from 3, and because Team USA abandoned all their principles trying to catch up.
It's still the same opponent though.
And it's an opponent that they proved they could beat without Embiid, comprehensively. If they had lost to Serbia without Embiid in their previous two encounters, then the argument of them needing him would stick.
I gather the premise of this thread is based upon Embiid not being a part of the squad in the first place, so it'd mean players like AD, Bam etc would need to have a slightly bigger role, which they can easily fulfill. And who's to say that if they did play Serbia again in the semi-final that Serbia would even be that hot from 3? You can't exactly replicate the same conditions of that semi-final without Embiid. It'd be an entirely different game. Who's to say that having AD in the starting lineup instead wouldn't have been more a difference maker on defense given his ability to defend all over the floor, leading to Serbia getting worse looks from behind the arc.
Embiid's buckets were certainly needed to keep their runs ticking over in that specific game, but as the homie said above, he gave up plenty of rebounds and was generally lethargic on defense (outside of battling on the block with Jokic). Bam and/or AD wouldn't have had to produce the same efforts on offense, they'd just need to provide more on the other end to stop Serbia from getting second-chance opportunities.