Reviewer responsibilities are generally volunteer hours insofar as the peer reviewing the article isn't getting paid, or compensated in any way for the many hours/day(s) it will likely take to properly vet a piece of work. Therefore reviewers eventually blow off some articles, choosing instead to do a quick analysis and hope the other reviewers perform a more thorough analysis. Apparently, in the case of this paper, all the reviewers blew off their jobs.

What John Dakota said, plus one more thing: I've noticed that papers written in the past ten years tend to be written by people who have poorer writing skills (to the point of sometimes writing nearly incomprehensible paragraphs) than papers published, say, 30 to 40 years ago. At least in my field and a few somewhat-related fields. It may be different in other fields.

It's possible that some of today's reviewers also have poorer reading skills than reviewers 40 years ago, and thus don't really understand what they are reading some of the time. Combined with a lack of time, they might simply skip parts they do not understand.