It will be interesting to read the book and see if the Montana is even referred to as Archer-class at all. Perhaps they just needed a smaller TOS-era ship, and Mr. Drexler used the Archer-class because the model was already available. Christopher already mentioned that it was cheaper to reuse an existing model when discussing the alien ships; the same thing could apply to the Montana as well.
Sorry for resurrecting this thread (and replying to myself), but I figured I should mention that the Montana was explicitly identified as an Archer-class ship in the text.
I don't know if this counts as a spoiler, but I'm going to spoiler code it to err on the side of caution...
The Montana was identified as a ship that specializes in a role of transporting Federation officials and representatives, IIRC. Which seems a strange role for what has previously been described as a fairly spartan surveyor. You'd think the Federation would have some other ship type more suited for use as a diplomatic courier/transport.