Can a systematic review only review one article? I got asked to review a systematic review in a peer-reviewed Pubmed-indexed journal. However, based on the author’s inclusion/exclusion criteria, they only review one article.

Isn't that peculiar? As a reviewer, how should I respond to this? Is this acceptable, only one article...?
If it’s a systematic review, it is weird that there is only one article to include. However I think it could happen. After having applied inclusion criteria to all articles first retrieved, only one was left. I also think there’s a lesson out of this issue that the authors of the review should address.