I found Sarah to have no issues and pretty much agree she had a right to defend her territory here. The first post was not meant to be offensive, but it perpetuated some misunderstandings that were damaging to Sarah's type. The connection with where it overstepped might be the problem for you, because different types are sensitive to different things.
But some things are global. Like if I suggested that ENFPs cared more about people and INFJs cared more about making tea for them, then you would find it insulting, and you'd feel the need to defend it. In a way that was the part that was found offensive by Sarah. The original post questioned her type as being weak on one of its strong points. One that would seem immediately obvious to her and a lot of types. It naively concluded something that seemed damaging to her. It might not have seemed damaging to you. But the fact it felt damaging to her is a good sign that it might have been damaging, because she was the one in the firing line, not the one guessing from behind the gun.
Most ISFPs don't conflict very often, well most Fi types don't. So when they do it is normally something they feel strongly about relating to effects on people. Take the opportunity to learn from it, appreciate differences and meet half way. Because these things often dissolve into arguments for no good reason.
Sarah had to argue the first part though, because a misconception had been formed that she felt the need to clarify. And most people who were N did not agree with her until quite a way into the thread. So it was useful for her to do. We learnt a lot more than if she'd been diplomatic and left it.