Yep. The only thing I have read to counter that (from the cite above) is the format of the LoR questions. In an elite prep school it is almost impossible to say “this kid is one of the best of my career”. A super strong student is more likely to stand out at a public, underserved school.
Doesn’t make up for the advantages of a prep school overall, of course. But I do think the AOs take the high school into consideration and try not to compare apples to oranges. It is just a lot easier to pick apples.