I am not sure why you think that way. If before Covid and Masks, someone felt unsafe or threatened on a bus due to the actions of another passenger who refused to give them some space, I am sure you would have advised them to leave the bus and get themselves to a place of safety. You would also, I assume, not expect them to immediately get back on a bus without some level of re-assurance.
You would not, again I hope, have made a judgement about them as a person, to why they felt unsafe or threatened. But now Covid is here and you feel it is absolutely fine to judge people for asking for a bit of personal space, and then when it is not given removing themselves from potential harm.
If the bus was empty or there was plenty of space elsewhere, he might have had a point. Otherwise, you can't expect to not have someone sit near you on a bus that is busy, mask or no mask.
Even if someone had sat next to him with no need, most people on public transport will automatically spread out as much as possible anyway.
