They can't use anything energy based
That's just silly. Everything is "energy based", from shoelaces to particle accelerators to photon torpedoes to chemical explosives. There's no particular reason to think that phasers would automatically be among the things the collectors can neutralize or utilize - especially if phasers were not part of the Promellian arsenal originally. And Picard seems to think of the Promellians as a kind of primitive forebears of Starfleet, achieving great things with less advanced hardware...