As someone who was also rejected even though I solved their challenges, I've got a feeling they are measuring the wrong stuff. The rejection said that I "didn't show the growth they are looking for". Maybe a fresh 2018 CS grad could do it? I don't know, I started out with qbasic on a 386...
In all seriousness, I had an interesting dealing with triplebyte regarding an internal job posting.
Took an assesment, was sent a message of acceptance, followed shortly by a rejection.
Felt 'weird'.