I am pretty young and it seems stupid to me too, this guy wanted to be a React Dev. If he was ever at work writing a binary search tree by hand, or writing his own quick sort function, then he would be wasting his time and creating a huge potential problem point in the code.
This guy claims to have worked ~6 days a week for 3 months not to become a great full stack dev, but to become a great interviewee. Seems kinda backwards to me.