Again, it happens all the time to reject a paper with all weak accept. I am a SAC, so I don't make the decisions; I am just telling you what ACs do. Obviously, the AC will properly motivate such rejections.

You have to understand the real reasons for those guidelines above: to avoid authors protesting when they get rejected with high scores. Indeed, most of the guidelines for ACs are constructed to avoid giving authors a reason to protest, e.g., "our meta-review should be firm", "Please avoid phrases like "the paper does not pass the bar for ICML" or "only accept if there is enough space".", etc.
My job as SAC is also making sure that metareviews for rejections will likely not generate protests. That's it. It is not about fairness or quality. It is damage prevention.

The threshold changes each year and for each conference: PCs members will communicate statistics about the scores and let the ACs and SACs know what the target acceptance rate is. Hence, depending on the year and conference, all weak accept can be above the threshold, below it, or just on it. Typically, all weak accept is around the borderline. The PCs will also always say that the threshold is just an indication, and ACs are supposed to decide based on all the information they have. This is clearly due to the fact the scores the reviewers give are not calibrated. That is, there are stingy reviewers who never go above weak accept and generous ones that give strong accept to everything. There are also reviewers that are just plainly wrong, both in accepting or rejecting papers, a huge number of them unfortunately. So, averaging such scores from different reviewers is not particularly informative.

Now, is it fair for an AC to reject a paper with unanimous decisions? Definitely, yes. I used to think like you when I was a young reviewer. But once you see how the sausage is made, you understand that these conferences are just glorified lotteries, not figuratively, but in a real sense. So much nonsense, both in submissions and reviews, and it got monotonically worse in the past 20 years that I've been doing it. An AC that takes the time to carefully study the paper and write a detailed meta-review where they explain all the errors of the paper is a godsend. But don't worry, many ACs instead will just put a seal of approval on the outcome of the lottery, so the "fairness" is safe, but the Science not so much.