Women aren't better at ultra marathons they are just more competitive as the distance increases.

Speed is mostly a matter of muscle mass and maintaining speed over time and distance is related to lung capacity and ability to manage lactate build up and men usually have more muscle mass, which means they can maintain more speed.

Women usually weigh less, have less muscle and have more bodyfat so they are ultimately more efficient in overall energy use which starts to become more important as the distance and time increases and the outright speed becomes less important.

Ultramarathons are not really like an marathon where everyone is running at their maximum sustainable speed, there are usually rest stops so women being efficient need less time to refuel on calories compared to men can keep or get going quicker. It's like having a more fuel efficient car that doesn't need to stop for fuel as long or as often compared to a faster car that needs to stop more and for longer at refueling stops.