He wasn't sure.
He moved to his proper mechanics spot after he the outcome of the toss. After he was on/near the base line, he was likely 97% sure of the error, but knew it'd be a reeeeeeally bad call if he was wrong (his only correct decision on the play). And in the time he took (likely around 2-3 seconds) to check, the drive started coming toward him, so he let it go. He clearly signaled for the GT call, which should have stuck, and the non-scoring team a TI under the appropriate basket.
So the crew screwed up the rule there. They should have all met together and discussed what happened and how to proceed. Each official gets a voice.
Also watch the U1 position after the toss. He takes a small step backwards after he sees black obtain the jump. He thinks he's going to the L spot. But when he see black go the other way, and everyone else too, he sorta fades away. He never becomes the slot, and stays out of the video range the whole time. So he likely knew something was up too. But I'd say less than 97%.
If he was 100% sure, he would have whistled play before the drive and set things straight.
It appears that the only official who had no clue is the tosser.
This is why every official doesn't just give the status quo thumbs-up. In your dead ball officiating, you assume responsibility as though you're the CC for setting up the play.
Examples: the U2 also checks direction, the FJ also checks penalty application, the line judge also checks the score for switching sides at 12 points (or whatever the rule is).