Alpha doesn't do the Core, Omega does, but the key is that both quarantine and influence squadron are Diplo actions, quarantine costs 4 and influence squadron costs 2 or 3, depending on the version, a D2 could spend 6, and thus the lesser influence, but that is to ignore raids, and a quarantine seems a bizarre action for a raid. Besides the fact that a decent diplo score prohibits being particularly great at raids, I'm not sure if a quarantined world, not producing any income, could actually be profitably raided. To go for a seizure would cost 3, requiring 7 DAP expenditure and thus a D3+, military conquest in the core doesn't seem very viable. Unless Omega doesn't intervene under quarantine. D3M2, sure, but I wouldn't expect a M2 to be planning military conquests in round 1. Big aspect being I still don't think they'd have a target they'd know to attack.

I'm am guessing that if this is one of us they're either doing something very sneaky and clever or just expending DAP they wouldn't expect to use to try it out and maybe screw up other player's income. They couldn't possibly have comprehensive intel on Brontes and they will need to ship their troops off before the vote is tallied I believe, so I'm doubting someone is actually planning a major offensive on the planet right now. This could just be someone's random feint, draw attention to that world or the core in general. I may be seriously overestimating someone's strategy here, but I can't figure out why anyone would want to quarantine a core world early in the game. It's the hardest to seize and the place you're most likely to encounter stiff resistance to a quarantine. If it were on Nephor I'd just assume someone was trying to drive down the cost on mining facility for auction there or prepping an attack, but nothing is bid on Brontes.