ADM press release from Dec 15, with a list of open stores and restaurants at YUL.

https://www.admtl.com/en/node/17681

You'll be waiting in the transborder area for your US flight, so you won't have many options, and none after 4h30 pm, at least not according to the list. The list might change if your flight is in a few weeks, so I would check ADM's website for an updated list, if available.

Also, there is a Tim hortons in the transborder jetty, and for some reason isn't listed as either open or closed in that list, which I find weird. Could be it's open and they forgot to add it.