It’s exactly what you said, it’s “close enough”. For the reason you think “Sword attack” is close enough to “Swordsmanship” to not matter is the reason why Stephen isn’t fired. Because a casual reader who isn’t like us that discusses it would not be confused by Stephen changing these words. They are close enough
also Oda and the Jump editors don’t understand English so they can’t fact check anything
I can’t say that you and all the viz readers technically misunderstand the narrative flow and structure of the story in a general sense from reading Stephen, he’s not that bad.
But you’re definitely missing out on a the more minute details. We all are. All of us English readers are missing out on details. So that’s why it’s better to use multiple translations (including even Stephens) and consult independent translators like Sandman and Puck (who is on this forum) and more.
Just know, for the sake of internet discussions, just saying “Viz said this” isn’t good enough. It’s fine as one source but not the “Ultimate source that silences every other source”