It could also be a situation, as the article says, where they have to exhaust all NYC-based options before they consider Metlife Stadium. So if either Yankee Stadium or Citi Field is available, it's either there or at our stadium, and can not be at Metlife Stadium.
Still seems like a situation that will (almost) never materialize. If they think they need larger than 25,000 capacity, they should just build it.
i can honestly see it for a Leagues cup final vs a popular liga mx team. But besides that even MLS cup is not really needed.