No Starfield is seriously going to dampen excitement for the showcase, already people are reacting negatively to Jez's leak, Lol. He does say "may not" so who knows, I honestly wouldn't mind if Starfield takes a backseat in this showcase but it shouldn't be removed entirely. Starfield is big enough for its own "Developer Direct" and honestly should get one even if it's in this showcase.
What I would do is end the show with a new trailer for Starfield and then announce that a Starfield Direct is happening where they'll deep dive into it for like ~30 minutes and ideally it's airing within the next few weeks, that'd be absolutely fine Imo, but since Jez has leaked this we're going to go wild with speculation until Xbox speaks, Lol.