... What I don't quite understand is what NN rockstar that exceeds the high bar isn't already aware of Tesla and what they're doing? Either they've applied and got rejected or have no interest in the job, right?
Being aware of Tesla and what they're doing does not necessarily mean you're either already with them, have been rejected, or are not interested. Maybe you're aware of them but need some added encouragement to apply. Maybe you already have a job and didn't think they'd be interested in you. Maybe you didn't realize they were hiring. Maybe when you did apply, they weren't hiring. Maybe you've seen their code or otherwise know something about it and you think they were on the wrong track and you didn't want to hitch yourself to that train, but with a re-write scheduled, now you see the possibility of getting in on a fresh start.
There are many possible reasons a bright young programmer might never have considered applying but might be interested in a hack-a-thon.