Well your guys are kinda right. But there's a deeper thing or 2 to consider.
1. S1 doing occasional group sex and even orgy isn't so off-brand. S1's brand is top idol, and top idol's status symbol is solo vids. So yeah S1's mainstay is solo vids (and vanilla). I only wish JAV can be more relaxed about it, once in a while letting two top idols be in the same vid or scene isn't gonna destroy their careers! In the good old day, the studios would put together some dream duo or even dream teams, just once in a blue moon, getting the fandom to a boil.
2. There's an epic drought of proper orgy across the board. It's not just an S1 thing, or brand thing. It's the industry-wide trend that just refuse to go away. Harem, especially POV harem vids are sucking up all the air from the group sex scene. The most painful-to-watch studio is ZUKO, orgy is their BRAND. Their early vids, starring both amateurs and idols, were awesome. But along with half the JAV world, they have gone to reverse gangbang or harems featuring one single testosterone-free limp prick.