Are you sure he'll actually be on TV? HBO are incredibly stupid sometimes, not showing a great match on an undercard isn't uncommon for them.
I'm positive, he's a world champ at this point. It was either Usyk headlining a card in Ukraine as usual or getting US exposure.
HBO has not announced that they're showing Usyk's fight. If they do surely depends on the opponent. He's supposed to be fighting some Bosnian fringe Euro level named Damir Belyo. Has a 22-0 record but against no one significant, beat washed up former world class smaller guys Epifiano Mendoza and Alejandro Berrio. I only needed to take a look at a minute of a fight of his on youtube to see he shouldn't be in the ring with Usyk. I hope HBO passes on Usyk this fight. Find a better opponent for March for an HBO debut.
HBO is no doubt using this upcoming fight as a try out of sorts. An exclusive contract would certainly entail fighting almost exclusively in the States like Loma along with unifications and an eventual move up to HW.