Yes it will allow games made with unity to run on stadia. It has nothing to do with unity, its whether stadia itself allows non-gaming applications and currently it has not been announced anywhere that they would. So your better off assuming it will be games only until officially it is stated otherwise. Stadia has not been marketed as anything but a gaming platform, and from researching there does not seem to be an offering or an appetite for it to be for other things. This wont be a platform where anyone can release a game, it will be vetted heavily like consoles were back in the day.