Heck, he attended a correspondence college for 2 years (Open University of Israel) before his family immigrated to Canada and then attended UToronto (an amazing university, but not significantly selective by any means).
Ilya is by definition an example of why heavily stratified systems are subpar for human capital development - they remove the opportunity to identify talent from a broad pool, because humans can change.
And as I pointed out from personal experience, the difference in outcomes between IMO and non-IMO participants when I studied CS at HYS was nonexistent - we all did equally well professionally as well as academically. The difference was we all had the ability to study and get guidance from the same professors if we so chose.
A rising tide raises all ships.