Not only Steyr's, but each and every double set trigger will fire the gun, if the rear trigger is pushed forward! This is a rather unknown fact, but is stressed by me during the next few days again, when I have to examine wannabe hunters during the German "Jaegerprüfung". As you note, in normal use the rear trigger is pulled back against spring pressure in "setting". When it is released by touching the front trigger, the rear one is accelerated foreward/up by the spring and slams against the primary sear and releases it. So of course it will release the primary sear also when pushed foreward by a thumb instead of it's spring.