I didn't know that either. The map I have from 2018 shows them as being in the Boston market. That may seem kind of odd, but Windham County really isn't "near" the core cities of any market, and viewership of this market or that has probably been largely driven by cable carriage. I suppose the viewers there would have to decide between watching TV (most of all news) from their own state, or from one of the largest markets in the US. It could, of course, be a "both/and" situation, but Nielsen assigns counties (or in rare cases, parts of counties) to one market and one market only based upon preponderance of viewing.
As a kind of aside, is there a Nielsen map available that is later than 2018? Any other kind of breakout?