I think you've misunderstood the slide. It says: If we do no other mitigations other than wearing masks (3 variants in the graph), this is what happens. If add

I think you've misunderstood the slide. It says: If we do no other mitigations other than wearing masks (3 variants in the graph), this is what happens. If additional stuff is done, then the risk goes down. It does not say how much, maybe a little, maybe a lot. But such other mitigations are unlikely to happen if the easiest, most effective and most accepted way, wearing masks, is not enforced already. That's why it makes sense to look at the case of masks-only.