Roughly speaking, because of the curvature of the earth standing at ground level, with perfect viewing conditions the average person can see the horizon as just about 3 miles.

Adding some height to your vantage point, 20 feet is about 6 miles. 70 feet is 11 miles. 400 feet is 26 miles. So being on a mountain that’s about 3000ft tall gives a sightline of 72 miles.

Boston is about 75 miles from the mountain so you can’t see any ground structures at all. The height of the taller buildings probably somewhere after the 6-8 floor or so is what’s being seen.

As a small edit: after driving into work today through downtown; I’d say the skyline we see this picture is more like building over 12 floors tall & up.