Question from a non-stutterer: how do I give feedback on a presentation in class if the presenter has a very bad stutter that makes him very difficult to understand?

Question from a non-stutterer: how do I give feedback on a presentation in class if the presenter has a very bad stutter that makes him very difficult to understand? Before corona, I attended a compulsory class in which each student needed to hold a presentation about a scientific paper. The students had to give feedback to each presenter, and there was one presenter that stuttered so badly through his presentation that I found it genuinely very difficult to follow him. After some consideration, I decided to mention it honestly in my feedback (our feedback did not count towards a grade), because he himself knew perfectly well how bad his stutter was and I would be fooling no one to pretend otherwise. After all, the course was not about the content of the papers but about the presentation itself. However, the person sitting next to me saw this comment (something along the lines of: 'It was a pity you stuttered so badly, I found it difficult to understand you.') and said I couldn't say that to a person. ​ I still wonder if what I did was indeed wrong, and what I should do in similar cases in the future (of course, if this persons grade had depended on my feedback, I'd have responded differently). Is it insulting to mention someone's stutter in a presentation if it hinders communication?