I did not know that there were common devs across Cinnamon and MATE.
I know that Mint wants to have app collaboration ( Xapps ) between Cinnamon, MATE, and XFCE. That makes more sense now.
These are then major GTK desktops that are not GNOME. GNOME apps are increasingly GNOME only so it makes sense for the rest of them to collaboration on a GTK experience that is not GNOME.