The OP wrote "I bet that at least one of your fellow students will wind up in a psychiatric hospital", not that you would know that to be the case. I doubt that the department head will tell people that student X is no longer in the program because of mental health reasons ... nor pretty much any other reason.

A friend of mine killed himself a few years ago and it took months before I could finally learn how he died, as no one would say more than "died too young." (A social worker friend of mine did clue me in that that's code word for a suicide.) Now replace that with "decided to leave the program" and how do you know what happened?

There is no need for precise data. The link is to a set of suggestions for people thinking of entering graduate school, made almost universally by people with PhDs. The OP objected to the lack of information about the dark underside of graduate school, and pointed out the likely survivorship bias.

I think you are unfairly raising the bar on the OP by calling for more rigor. If you think it's anecdotal fear-mongering then you must also object to the original responses, as their individual experiences are also anecdotal, and perhaps even anecdotal whitewashing. Their advice did not include any real evidence either.