I'm not sure that what Alexander was doing in that one instance nor what this author is talking about are "steelmanning". If in order to "steelman" your argument I also have to accept every point where you beg-the-question then where can there possibly be any argument? I don't think I have to accept the argument that every fetus has an immortal soul to steelman a pro-life position. But I do however have to accept tha…
Yes — when you’re presenting the “steelman” version of someone else’s argument, you have to use their premises.
You outline two distinct arguments in your question:
1. A secular pro-life argument
2. A religious pro-life argument
Substituting a secular for religious argument is a strawman: you’re responding not to the best form of their argument, but to another argument they didn’t make.
In the case of a religious argument, the “steelman” case would be to articulate the religious argument to the best of your ability — and then dispute it by discussing which of the premises you disagree with and why. In your example, disputing the premise fetuses have souls.
The reason for “steelman” arguments is because that allows you to directly dispute the disagreed upon premise by clearly defining it and showing how it is integral to their argument.