i might be showing my age here, but i'm pretty sure the "muselist" terminology actually evolved from soulbonding crap back in earliest days of lj... (like from around 2001-ish) which dovetails nicely with the "headspace" shit they're apparently using now.

in ye olden times, people on lj would have "muses" (which was what they called soulbonds back then), and would branch into the rp communities to let their soulbonds interact with other soulbonds and rp characters. the "muse" and "muselist" terminology started to catch on, and for some reason non-soulbonders started using it.