Source on that claim? Here's a comment chain where the author admits that his library is noncompliant: https://probablydance.com/2017/02/26/i-wrote-the-fastest-has.... Also note the complete lack of a standards test suite.

This library uses Robin Hood hashing for speed. As my original link explains, standard implementations use chained buckets so that users can cache lookups and to have better worst-case performance: https://news.ycombinator.com/item?id=9675964