> the best possible form of their argument would probably involve dropping that -- even if it's actually a significant motivation

If a point is so significant, then the result of dropping it would no longer be “their argument.” Doing so would not qualify as steelmanning by any definition I’m aware of—it would be a different argument altogether.

To steelman is effectively to practice the principle of charity, by not assuming irrationality when a plausible rational view exists that fits the same arguments. But if one’s opponent has made an irrational argument, no assumption is necessary!

> I think this may depend on what definition you're using

What definition do you use? It doesn’t match how I’ve ever seen the term used.