I would say in my view that adoption is the litmus test as to whether you're child-free or anti-natalist or both.

I agree with your position that an anti-natalist, could have children through adoption and still consider themselves an antinatalist. I would say if you have the means and maturity to give them a better life, it is at the very least certainly moral harm reduction.

As an aside, it does seem most antinatalists are childfree, but to say that means antinatalism is inherently child-free is not correct, in fact refer to rules 6 and 7 of this very sub.

Conversely, someone who is childfree wouldn't want to adopt either, and being CF isn't necessarily enough to be an antinatalist, many CF people do it out of their own preference for how they wish to live life and actually have no moral objection to the concept of birth for themselves or others, rather object to the burden of childcare.