To be fair, if you wanted to do a performant implementation, this is probably a more expensive implementation. The expected response is generally for you to iterate over the string and construct a count map of how many times a character appears in a string, and then iterate over the keys of that object and have at most one odd number in the values of the count map.

The interviewer probably should have probed you about performance though if their intention was to judge you on that (they're both O(n) time complexity, but split, reverse, and join are generally more intensive).