I think that's the "not two months before travel date" reference as opposed to at the gate..

I have never heard of anyone with a confirmed reservation (points or otherwise) having their reservations cancelled....unless, as was mentioned, there was a equipment issue but even then they should have a plan B and present the traveler with options

Edit: Just being curious and having no idea what day the OP is flying on I checked every Saturday in January BOS/SXM and found at least two/three flights both ways every Saturday which I believe is the most popular travel day to/from SXM

With that said there is no reason why the op wasn't put on an available flight