The main reason the Harry/Cho argument might have seemed uncertain is that the series is often about changing perceptions where Harry himself is often wrong, and can even be wrong over the course of several books. Since the series still had two books to go after this argument, it was open to debate whether we were supposed to be siding with Harry or Cho or see that they both had a point.
In Book 6, Marietta makes one brief appearance at the start of the next school year. Harry sees she still has the jinx on her face, smirks at the sight of it, and walks away. Clearly, he's had no change of heart about her or his argument with Cho. He still resents what she did and he's glad and amused to see that the jinx is still on her several months later.
In Book 7, Marietta makes no appearance and no mention is made of her current condition. The only time she's mentioned at all is when Hermione, the very person who scarred her, refers to her in passing as, "that stupid Marietta". Cho makes one final appearance where she's all smiles and good cheer towards Harry, with, as noted, no mention of her friend's current condition.
Shortly after the release of Book 7, Rowling gave a fan interview where the fans could submit their own questions to her. What happened with Marietta's jinx was one of the first questions asked. Rowling answered that most of it faded with time, but it still left a few scars. Rowling then added, "I do so loathe a traitor!"
With that comment, Rowling's intentions became 100% clear. To her mind, Marietta was nothing but a loathesome traitor who richly deserved what she got and Cho was being foolish for sticking up for her. We were all supposed to fully agree with Harry's arguments and dismiss Cho's arguments out of hand, hence a straw argument. We were supposed to be just as amused as Harry was to see the jinx was still on her face the next year. We weren't supposed to care or feel sorry for Marietta being permanently scarred for this one bad thing she did. Rowling didn't think Cho would resent her friend still having scars from the incident two years later.
Even before the interview made Rowling's intentions clear, there were hints in the book that this was a straw argument. Harry got the last word on both the points that were discussed on this page. Right after Harry defends Hermione's jinx as brilliant, Cho sarcastically refers to her as, "darling Hermione", expressing the jealousy for her and Harry she'd expressed earlier, something Cho's demonstrably wrong about since Harry had never shown any concious romantic feelings for her (and by the end of the series, he never does). Having her be wrong on this, implies she's wrong about everything.
It's debatable how much of a strawman Cho's supposed to be in general. But for this argument, her statements were clearly meant to be straw statements we weren't supposed to agree with. The fact that many of us did think Cho had a point, even the fact Rebochan didn't realize this was a straw argument, is an indication at how much Rowling failed in making this a straw argument.