My kids are in public school, not Catholic school. I was comparing a system that was designed specifically to produce college students to the default for people in my area of Chicagoland, and finding that the default was if anything more rigorous than the selective private school.

The point isn't that they're "doing well". It's that the coursework they're assigned is rigorous. You could also have students doing poorly on such coursework, but that would also contradict the anecdata in the main article.

I don't know if this means anything to you, but, to be honest, I somehow got the same impression from the comment in question.