That hasn’t borne itself out in my area.
My experience is all I have to draw on, of course, and in any case the plural of anecdote is not data. But I will say that my son (Sal, excellent GPA/scores, nationally ranked/ competed internationally in his EC) applied to a handful of schools in the “super selective” range as well as his match schools. After he was denied, we heard from several people that the only kids in our (large metro area) district who get into those particular unis come from one of two schools (one IB, both highly affluent, which ours is not by a long shot), at least for the last ten years.
Discussions with the GC and a friend who does college counseling locally tends to bear that out. Their impression is that, like someone mentioned, the known quantity is preferred.
Could be coincidence, could well be S was denied for perfectly logical reasons unrelated to his high school. He’s happy where he is so I don’t lose sleep over it (and in one case feel he dodged a really big bullet). But one does tend to wonder. And I wish I’d known before I spent money on application fees!