In a Private group (especially MLS):
- there are clearly defined Admin
- there is a group state, in the sense that if you're not up to date with it, you can't encrypt messages any longer
So it's not really appropriate for one-off communication.
In Big-Tech land, the third party server is the hidden owner of the communication channel between 2 profiles that "DM" each other. Here we don't have that. So either you create a Private Group (with admin etc) or you send one-off messages that, imo, we should better call "Mail".