Just listened to Katchalov interview on the pokernews podcast and he talks about the hand 41 minutes in. Listening to the perception of someone who was at the table, makes me lean even more toward 10s9s.
Katchalov said that Morgan was looking straight at him on the turn as if he wanted to tell him something and his eyes light up. He then started to look around as if he wanted to have people's attention.
He also looked around for attention after Smirnov overbet the river, like he was very confident about his hand and didn't need to study his opponent.
Smirnov told Katchalov that he overbet the river to get a tell on whether he had the straight flush or not. He thought that if Morgan reraised all-in with confidence after he overbet the pot, it would be a good indication that he had the straight flush.