Brimson switched with the blessing of the ARL, which was a factor in the IRL allowing him.
There is also the fact that Brimson really has had no opportunity to represent the Kangaroos since the Nines tournament. While Ponga pretty obviously would have been picked in '23 and '24 if he made himself available.
I think the ball is really in the ARL's court. If they say nah, we wanted to pick him in 2024 and don't give it their stamp of approval, is the IRL really going to go against the ARL? They are really just a puppet or the ARL anyway, aren't they??
Maybe that is outweighed by Ponga having a higher standing than Brimson does, and the threat of pissing off a big star that is already a Union-flightrisk? I just don't think the Brimson decision totally guarantees Ponga can switch, even if he wants to.