This is a good tune-up fight for whoever he is going to face on the winner of the Fury - Wilder match, but we all know Joshua got a huge upset from Ruiz, and Usyk is a much better fight than Ruiz, so Joshua must be very careful not to get caught in an upset, he is the heavy favorite but Usyk is an undefeated and can pull out an upset.
Just like what others been saying about upset is likely and you are right that Usyk is much better than Ruiz when it comes to technicalities and speed and if AJ's fighting style hadn't changed wayback then most likely he would be KO'ed again this time.
Ruiz KOed Joshua in their first fight and Joshua UDed Ruiz in their second. It may look like Joshua did not completely avenge his first loss but if we look at the fight, it was a total domination, a show of boxing skills, from Joshua. As a matter of fact, 2 judges scored the fight 118-110 in favor of Joshua, meaning Ruiz only won a couple of rounds. The other judge scored 119-109 in favor of Joshua, meaning Joshua won 11 rounds and Ruiz only 1.
Upsets do happen but they do not happen most of the time. If Usyk's only hope is an upset, then he'd most likely lose. I doubt Usyk could KO Joshua. It's a matter of luck. On the other hand, Joshua has got what it takes to defeat Usyk, even KO him.