You’re right. But what are you trying to prove with on the spot coding challenges? That the candidate can implement a hash table from scratch in 15 minutes with O(1) performance?

Is that something they’re going to do on the job? Very unlikely. So why are you not only testing for it but testing for it in a fake time-pressured stressful situation that also rarely exists on the job? (Yes , we all have deadlines but when are they ever 15 minutes on the spot?)

Perhaps you should have entered the education field where you could give such exams day after day to your heart’s content?

People giving on the spot coding exams, or worse whiteboard coding exams are just assholes. They're trying to haze interviewees and make themselves feel better. There's zero actual utility to such tasks in an interview.