Who's telling them they are inherently bad?

It's a good question. The best message here might be "we've made the decision to prioritize fighting marginalization and we think this is the most effective way to do it -- nothing personal white guys". But it's a delicate message to send, and some people will never accept this as an appropriate way to hire: "why do I have to be completely shut out to make inclusion possible?"

UBC? I mean they're saying it would be bad for straight white men to get a tenure-track position. There's no difference between that and "we think you're bad and don't want you".

I don't see that they're saying that. I think they're saying it's very important for UBC to get some marginalized people into these positions, so they're making space for such a person. It doesn't mean white men are bad. (And technically, FWIW, the passage doesn't explicitly count sexual orientation as a dimension of marginalization.)

By definition it does mean that - they have identified a problem (something bad; too many white men) and a solution (a way to make things better; get rid of them).

Clearly it's painful to accept this, but banning people from positions on the basis of their race or gender is racism, hateful and always has been. Whatever their motivations are or are claimed to be is irrelevant. The claim that it isn't racism if it's against "powerful" people i.e. whites is a lie.