I don't think he is thematically related in any specific sense. But he was "socially" connected to the Transcendentalists -- and he was part of the same broad intellectual renaissance.
This is a really out of left-field metaphor, but it's sort of like how the Police and Dire Straits were considered "punk bands" in the late 70's even though they were completely non-punk. But they played in the same bars as the other punk bands, and they were new bands in a time when most new bands were punk bands, and so they are generally grouped with punk bands. I'd say it's in the same way that Hawthorne is often grouped with the Transcendentalists.