I would say if you injected COVID into people, very near 100% would become infected unless they had existing immunity. An injection of a pathogen at a viable level is much more likely to proceed to a viable infection than just being exposed to it i.e. being near someone who has it.
But it seems a lot of us do have existing immunity. Maybe as much as 80% of us.
Though, yes, at the levels they used in this trial (about a billion viruses per injection, it seems) then I'm not sure if that helps - the usual immunity routes may have difficulty coping with that quantity of virus. Of course you wouldn't encounter anything like such amounts in a normal initial infection.