He can't be mistaken, the story is over. Also, Momoshiki would have been a huge threat to Kaguya. Kaguya's main ability and main ace in combat is her immense chakra level. This ace wouldn't even matter against Momoshiki as his ace in battle is the ability to absorb and use the chakra of others. This is why the White Zetsu were created, as they also have this power. They were created to level out the field between the two, this single power alone made Momoshiki a threat, by using White Zetsu, they would be able to cancel out this and even out the fight, ensuring Kaguya's victory.Sasuke deducted that the threat was Momoshiki and Kinshiki, but he may be mistaking.
The guy that Boruto, an accademy student, killed shouldn't be that big of a threat to Kaguya.
How could white zetsu help Kaguya to defeat Momoshiki and Kinshiki?
The idea that the threat is not Momoshiki's arrival is growing.
In most ways, Momoshiki is a huge threat to anyone who relies solely on chakra and not tactics. (Pure power, no brain in other words.) Kaguya being the prime example.