You cannot use the posted contact anyway!
Sorry but I don't understand, I've used (on a shelly1) the "external contact" parameter with the door sensor (third picture in the link) and it seems to work properly, did I made a mistake ?
Or did you mean I can't use a shelly2.5 with the addon ? (which I know, and which I find bothersome)
(my first language is french, so maybe I don't understand something)