I hardly think this is a "terrorist-like demand". For one, Ross Brawn has already touted the idea of accomodating independent suppliers in the 2021 regulations. Secondly, it's not the first time Formula One has tried this - the 2010 regulations were designed to get the likes of Cosworth into the sport, but the failure to agree to all of Mosley's reforms made it impractical in the long run. So I'd prefer to think of this as Marko wanting The Powers That Be to do something more than just talk. Red Bull don't have a long-term supply deal in place, and having an independent supplier would give them options, but they clearly don't want to waste three years working on something, only for the sport to settle on the status quo for another seven years.
Besides, Ferrari (used to) hold the sport hostage all the time. It was Luca di Montezemolo's favourite tactic - threaten to walk and hope that the public backlash against a Formula 1 without Ferrari would cow the FIA into submission. It's one of the reasons why Ferrari get $90 million just for showing up, and they get it before the World Constructors' Champions get their prize money. Marko might be thinking of Red Bull first, but at least an independent engine supplier having a feasible chance at success will benefit the sport, unlike Ferrari's threats to walk away.