It's not a bluff, and of course the leagues will cave and slurp up the solidarity payments.
And eventually the Super League clubs will wonder why there are any solidarity payments at all, and why that 300MM or whatever isn't going to the members of the Super League. And why they spend 90 minutes playing Burnley instead of 90 mins playing Real Madrid -- again -- for more money, prestige, viewers, etc.
And so then they'll leave the domestic leagues. Because what will they need them for? Nothing. Literally nothing. And the solidarity payments will disappear. As will the leagues as we know them.
The solidarity payments are hush payments for current non-elite club owners, and what's left will ultimately be spent on the rope the leagues will use to hang themselves.