This claim, and study which underpins the claims, was covered on the BBC More or Less Podcast last year. One of the main problems with the stat is that in ultramarathons there are very few women, but those that do enter are typically professional ultra-athletes, not "have a go" females. Whereas the field contains many "have a go" men who are less professional and less fit than the professional ultra-athlete men.
https://www.bbc.co.uk/sounds/play/p0hg2764 (9 minute episode).