I'm pretty sure this is the first time it's been explicitly stated, but it's always been theorized (by myself and others in this forum, anyways) that that's how it was gonna go down. For one thing, if it were to continue whittling them off one-by-one all the way until they got to five, that'd make for a reeeally awkward final few rounds. (And I mean awkward both in-universe and narratively speaking.)
My guess? 5-on-5, Team Mimi/Ibuki vs Team Ritsu/Hitoe. (Or maybe, if he feels like being extra spicy: Team Mimi vs Team Ibuki.)
Ha! I don’t mind even if they lose, but not putting Mimi and Ibuki together in the final team sounds wrong especially if the story ends right after. But the author can do whatever he wants of course.
I still feel a final 5 v 5 makes sense from group building standpoint, but doesn’t from business/agency POV and could be an awkward finale from reality TV perspective if the teams are far apart in quality.
So, the compromise I can see happening is the final round would be something like a pseudo-debut for both units. After the results are out, the winning unit get to be produced by Aria (as initially promised), the losing unit will end up being produced by someone else.
Since otherwise letting them go would be a horrible business decision because all other agencies would be more than happy to sign them up.
Personally, rather than a 5 v 5 I think one possible (or better IMO) final round would be multiple mini-contests where Aria gets to observe the chemistry of each contestant with a few others. (Arguably, the rounds so far has this aspect partially in mind.) The 5 that form the best group (have best chemistry) win. This is because presumably at that point, the remaining contestants are all of high skill levels and good foundations. And, the goal is to get the best unit.
Further this emphasis of the final screening based on group chemistry nicely ties in with the beginning of the series with Mimi and Ibuki not meshing with their original respective groups.