I disagree with your disagreement (opinion) and I'll explain why (evidence).
I definitely don’t think the burden should be on the reviewer to defend their review. Since it’s subjective anyone reading can draw whatever conclusions they want, as you did.
Contributing to a public site DOES impose a burden on the reviewer. The expectation is that reviewers are altruistically providing opinions and facts that will help others make decisions about restaurants, and that requires the reviewer to think of OTHERS, not just themselves. By posting on a site where people look for advice, one should provide advice that will be useful to others, and that may include having to explain WHY they were unhappy with a meal, so that others can understand whether or not it might apply to them.
Maybe it was a discourteous waiter that triggered the reviewers discourteous reaction. After all, at a high end restaurant you might expect the waiter to remain professional even if the customer wasn’t.
Yes, I would have that expectation.
The reader of reviews needs to read them critically, considering to what extent any particular review might or might not apply to them. Although I can't know whether the customer was at fault, the wording would lead me to rely less on this particular negative comment than I would on a more nuanced negative comment.