She is not stuck in Montreal because of the closure of the Saint-Lawrence Seaway, she is stuck in Montreal because of the ice conditions in the St. Lawrence River and Gulf. They just don't want to have to drive her through any ice even though the River and Gulf do not close in winter - are open year round - and are actually opened right now (though, between Montreal and Les Escoumins, traffic is restricted to daytime only during the ice season). I am not surprised, because the speed of those ships is gained at the expanse of hull thickness, so they are, in fact, quite fragile as hulls go.
For those not knowledgeable of the nuance (and neither the US Navy spokesman nor the Guardian appear to be), the St Lawrence Seaway begins just West of Montreal harbour, at the Saint-Lambert lock and goes only to Iroquois lock, which gives access to Lake Ontario. That's it! In Montreal harbour, you are OUT of the Seaway and into the River system.
And she made it through the Seaway before it closed. She arrived in Montreal on Christmas eve (as reported in the Gazette) and the Seaway closed on December 29, 2017 for this winter's maintenance period.