GTK4 and libadwaita forms are different from each other. You can still theme an application written in GTK4 with existing themes. However, adw forms will be used in GNOME specific applications. In my opinion, since MATE is based on GNOME 2 user interface standards, libadwaita is not used even if applications are transferred to GTK4.
Also, useful applications (like Disk Utility) made for GNOME 3.x should be forked for MATE with the normal menu and titlebar instead of the header bar and the useless appmenu before being ported to the libadwaita.
Additionally, I believe that there will be a method that adapts existing themes to libadwaita in the future.