I'm guessing a majority of the most committed pro-life activists aren't motivated by a concern for potential people; they actually believe a foetus is a person, not just potentially a person.

Thanks for catching this. The words "person" and "potential person" were just intended to be labels, but I ended up unintentionally sneaking in connotations. Let's imagine that in my counterfactual situation no one is doing that either intentionally or unintentionally.

one [pertinent biological fact] is that there is no such thing as a soul

I don't disagree, but I do think religious people might claim something similar to, "even though the soul is not a physical thing, it still exists in the sight of God".