As a native English speaker, but not an academic researcher, I would have naively done the same.
To preface: I am speaking in ignorance of the mechanisms of credit and advancement in your field, so I'm undoubtedly overly harsh. I'm not even trying to be fair, because I admittedly don't have the knowledge to do so.
I know academia is a different world from private sector industry, but I would think if the point of a confidential review is for you to do anonymous work to improve someone else's credited work, and their work was improved by incorporating your feedback, you would be happy with the outcome, or else why are you participating in a confidential review process in the first place? There are different mechanisms for publishing words that you want credit for.
When someone incorporates my feedback on code or documentation word-for-word, I might in the worst case be suspicious that they are trying to get my approval without engaging with my criticism, but in most cases I'm flattered that they respect my idea enough to put their name on it. Although, in my world, putting your name on something is more about responsibility than credit. The command is called "git blame" and not "git credit", after all.
Wanting them to incorporate your feedback, but also wanting them to make some change to the wording to avoid plagiarism, smacks of how freshman essays are graded, not how real work gets done.
Like I said, I'm not trying to be fair and don't have the right background to be fair to you. I only speak up because I grew up in an academic family and know that there is a presumption that academic work is more idealistic, more altruistic, and less mercenary than private sector work, and I think it's worth pointing out when the reverse is true.