Yeah, pretty much. The vast majority of mainline HM games were developed by the same studio that was once Victor/Pack-In Video, but there have been personnel changes over that time, of course.
Natsume developed/produced a few of the spinoff games bearing the Harvest Moon name but they didn't really work directly on any of the mainline games from the ground up--they might sometimes handle the "girl version" or a port to another console, that sort of thing. I think they might have made one/all of the Game Boy games? I forget. Lost Valley will be the first one that's 100% theirs.
Yasuhiro Wada, the "godfather" of Harvest Moon, left Marvelous a few years back; he's since started a studio called Toybox and made a HM-style game with Natsume called Hometown Story, but it didn't get a positive response and one has to wonder if they'll ever make a sequel.
The Rune Factory games were all developed by Neverland (the Lufia devs), and the other spinoff game Innocent Life was developed by a company called ArtePiazza. Neverland recently went bankrupt but Marvelous still owns the IP and a lot of the old team is at Marvelous now, so it's bound to continue. Innocent Life was a flop and probably won't be coming back anytime soon.