Seems to me that exJW's who reject the Christian label being attached to Jehovah’s Witnesses do so because they want their own move from one denomination of Christendom to another to seem more significant than it really is.
Well yes, that's one way of putting it. Another way of putting it is that ex-JWs who still believe in the Bible take seriously the idea that JW doctrine may be defined as heretical.
Just as atheist former JWs who want to include JWs as Christians are basically saying: Christianity isn't true, it's all varieties of non-sense anyway, so why not simply let them all be called Christian? What does it matter?
If you take the Bible and belief in God seriously then obviously it's going to matter a lot whether groups like JWs teach false doctrine.
If you don't take religion seriously then you may view it as mainly a matter of historical or cultural labelling, and correct doctrine is neither here nor there.
I think it also depends on context and who is asking the question and why.
For example if comparing JWs, Krishna Consciousness and Sufism, and someone asks what sort of religion JWs are, then "Christian" would be an accurate and enlightening answer in that context, because they are firmly within the Christian historical tradition, rather than Hindu, Islam or whatever.
But if someone asks a Protestant minister whether JWs are Christian that's a completely different context and may result in a different answer, with reasons for the answer. That doesn't mean the minister would be lying if he said JWs are not Christian, but he is answering a different sort of question with a different purpose in mind.
A different discussion again will result if the label "Christian" is being discussed in a moral sense, and whether JWs live up to the name Christian, if it is taken in the everyday sense to mean a standard of kindness and charity toward others. Some will say JWs meet that definition of Christian and others will say they don't.
Atheists will strongly dispute this meaning of "Christian" as kind or good of course (although it's there in the dictionary, and everyone understands what it means) and they are not interested in correct doctrine or what God supposedly approves. So of course from that perspective it makes sense to say that historically JWs come from Christiantiy, they rely heavily on the tradition and they claim to be Christian, so why not call them Christian?
It all depends on your perspective.
