You are not correct. We have actual experience, not hypothetical one. I had a kid in the same magnet. Kids were heartbroken when out of 50 only 1 was accepted to Umich and 49 waitlisted. No college can accept kids only from the same magnet program. They indeed do it by region but when you have all top 50 kids from the same school, they take 1-2 from this school and then 1-2 from another etc.
For reference. SAT will not be a problem for this kid. Prediction is 1560 or higher. Kid will not be NMF due to very high cut off in our area.