Not to be dismissive, but you reviewed hundreds of abstracts. A top conference in this area receives hundreds to thousands of papers. So there is a non-negligible chance your submission trickles down to some PhD student who is still learning the ropes of peer review. And then, there is a non-zero chance they won't fully understand the fine print and give you a thumbs up.

Again, this does not seem the case here (this one was likely uploaded to ArXiv after being rejected) and I thnk the opposite (good papers being rejected) is way more common in academia, so I wouldn't frame it as "low standards". Just an unfortunate outcome of "publish or perish" in a conferences-first community that highly values a small subset of all venues.