I think you're confusing limited participation and what such a small group of people doing these events means for single individuals to "win" an event. Women are more like to win in these events then others because there is less competition overall so you get more anomalous results rather then the male biological differences stop dominating the outcome.

You are right in that "strength" isn't the dominating factor for these events or why males go so much faster/farther but rather VO2 max and for peak athletes males normally maintain a good 10% lead due to biological factors.

The male vs female 100 meter:

9.58 vs 10.49 = female record is 9.5% longer to run

Male vs female 200 meter:

19.19 vs 21.34 = female record is 11.2% longer to run

Male vs female 50km

2:38:43 vs 2:59:54 = female record is 13.35% longer to run

The difference also doesn't really change once we start going really long either

6 hour: 98.5km vs 85km male ran 15% farther

12 hour: 177.410 vs 153.600 male 15.5% farther

24 hour: 319.614 vs 278.621 male 14.7% farther

48 hour: 485.099 vs 436.371 male 11.17% farther

6 days: 1045.519 vs 928.577 male 12.6% farther

https://en.wikipedia.org/wiki/Ultramarathon scroll down to the male vs female records.