“If this student was really that high-achieving and indeed in-line with Questbridge requirements, I’m sure that he/she would have matched immediately in the early round (but he/she did not match to anything).”
That you think this, indicates that you don’t understand how QB works. Many of their candidates don’t get immediate matches! Many go to the RD pool. Even then some never get a direct admission. I personally know a QB candidate who only managed to make it onto one waitlist, and ended up choosing a different non-QB institution rather than waiting all summer to find out if the QB place would come through.