I think that's debatable. People do shift which points they bring up and how they frame things while still feeling that their essential argument is unchanged.
> What definition do you use? It doesn’t match how I’ve ever seen the term used.
That's probably the rub here: the idea of "best possible form of the argument" leaves a fair amount of room for interpretation. I don't have some single authoritative source for my conception of steelmanning, I've just seen people use the term and I've read it explained in various places a few times.
In my defense, here's a source that agrees with my conception: https://debate.fandom.com/wiki/Steelmanning
> Steelmanning involves arguing against the strongest possible version of the other side's argument, whether they made it or not.
And here's a definition that's rather ambiguous: https://www.lesswrong.com/tag/steelmanning
> Steelmanning is the act of taking a view, or opinion, or argument and constructing the strongest possible version of it. It is the opposite of strawmanning.