I am not an incel, per se, but my decision to not procreate would ultimately place me in a similar category as an incel, as neither of us would produce any children.

Now, getting this out of the way, the reason for an anti-natalist, who is one either by choice or not, to support a movement or philosophy that is ultimately pro-natalist, or has such attributes, is simply out of safety. The comfort to practice my religion and spiritual system comes from living in a first world country, that isn't torn apart by civil war and ethnic/racial strife. It would be significantly more difficult to do if I had to constantly fight off inbred fanatics. So preventing your nation from becoming multicultural, and the tribe that your vessel was born among from becoming a minority in their territory, earthly boundary, is something that everyone should try, whether or not they are concerned with worldly matters.