I did a review of someone's code submission once where they pretty much wrote a bespoke JQuery implementation to handle the super-complex task of incrementing a number in a div when a button is pressed. I don't think they got the job. I felt a little bad for them, but a valid part of those coding interviews is "Will this person go off and do something crazy like write their own half-baked React instead of just using React?". Showing that you know how to write code as simply as possible is a big plus.

I have yet to administer or discuss interviews where using an api was an acceptable answer. Granted my experience is more on the engineering side but it seems odd to me to not expect someone to implement their own algorithm. If they had 60 seconds to answer a multiple choice technical question on a prescreen online test, maybe. But it seems that person demonstrated their understanding on a technical level that should have been expected and explored?