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I was accepted unhooked to Stanford, MIT and Ivies in spite of having a HS GPA/rank and combined SAT well below the median for HYPSM… type colleges and having ECs that were nothing impressive beyond a regional level (placing well in some regional math contests, math and quiz bowl type competitions between schools, etc.), unless you count video game related accomplishments, which I don’t recall whether I mentioned on the application. I think the biggest reason for the selective college admissions involved taking post-AP courses at a university, much like you described. However, it was more than just the act of taking DE classes. I think the reasons were also relevant, such as demonstrating intellectual vitality, taking advantage of available academic resources in area, what was accomplished in those classes, etc.
I was in the unique situation of being at advanced level because one of my 9th (might have been 8th) grade teachers recognized that I’d do better with independent study at my own pace than going at the class pace. I worked the teacher and my GC to support this, and did quite well while studying textbooks + other material independently instead of going to class, and going at double or triple standard class pace. I also worked with the GC and a nearby university (parents were minimally involved in all of this) to help setup a half-time type schedule between the HS and university where I’d take morning classes in HS, then drive to the university for afternoon classes. As far as I know, nobody form my HS had done anything like this before. The classes I selected at the university weren’t just the usual continuing math sequence to multi-variable calculus and linear algebra. Instead I took several classes in electives for no reason other than they seemed fascination to me. For example, I particularly enjoyed a class called biopsychology and behavioral neuroscience. I also received A’s in all university classes taken during HS. One of my professors mentioned that he was quite surprised and impressed that I was a HS student when I asked him for a LOR and seemed quite enthusiastic to write it.