Using Sonoff Dongle E just for Matter possible

I have a Sonoff Dongle P running my Z2M Zigbee network.

I also have a Dongle E in my drawer which I flashed with the Open Thread Firmware. Connecting it to my HA server I can see it in “Hardware”.

Trying to install the Open Tread Addon and starting its configuration I can select the Dongle E but the moment I hit the save button (which also seems greyed out) the selection of the stick is gone. Also tried it in YAML with same result.

Am I doing something wrong or is my setup not possible?