The term "steelman" arose from people who misunderstand the term "strawman". Such people coined it out of the idea thinking that a strawman was an an attempt to make an opponents argument look weaker than it is, while a "steelman" elevates it to it's highest state before attacking it.

In reality, a steelman is just another strawman. A strawman was never simply a matter of making your opponent's argument look weak, they're about making a separate argument that your opponent isn't even arguing, and attacking that to make it look like you're winning the argument while not actually addressing the opponent's actual argument/position. A steelman does the same. In other words, they're about fabricating an argument and making it look like it came from the opponent, before attempting to prove it fallacious. They're both failures in logic - a fallacy of relevance.