So, since when is deconversion a kind of evangelism?
That, I think, is the key point here. Evangelism is the process of converting people to a particular belief system. The word itself refers to Christianity, but I suppose it could refer to any belief system if you wanted to generalize it. The thing is, atheism isn't a belief system at all, anymore than the statement "I dislike apples" tells you anything about what a person likes. So how, precisely, do you convert people to atheism? Deconverting them from Christianity or some other religion, sure. But atheism in and of itself doesn't provide a replacement belief, anymore than saying, "I don't believe in gods" tells you what a person does believe in, or if they believe in anything.
So, I would hardly call Peter Boghossian an atheist evangelist. The question is, what is he trying to promote, not what is he arguing against. And according to the Amazon blurb posted earlier by magicmiles, what he's trying to promote is reason and rationality. So if he's evangelizing for anything, it would be those things. It's certainly true that he's arguing against religious belief, but this is no different than a Christian evangelist arguing against, say, Islamic belief or Hindu belief. But you don't refer to them as an "anti-Islamic" or "anti-Hindu" evangelist, you refer to them as a Christian evangelist. So, by the same token, you don't refer to someone who's pushing reason and rationality instead of religious belief as an "atheist" evangelist.