The Elves like Marienburg because it's easy money for them. They don't need Marienburg or depend on it. It is of minimal interest to the dominating motive for High Elf foreign policy; securing and projecting force over the entire world be it against Dark Elf, Orc, or Chaos. It isn't a military anchorage for them, or an ally they depend on, it's just easy money. Ulthuan stepped in with a few elf mages being eligible for being hired on for mercenaries last time when the Empire went back for it's clay. This isn't the Empire coming back for it's clay, it's the Empire being threatened with a massive blockade/embargo that threatens it's existence and proceeding to beat the ever loving shit out of Marienburg if it can.