I think they’re basing that claim on some buried women exhibiting archery and horseback riding strength in their skeletal structures. I doubt they routinely fought in the army, given the lack of contemporary accounts observing any women in the Mongol army.

This contrasts with, say, the ancient Sarmatians, where we see female burials with actual weapons of war, and have contemporary accounts that women did fight extensively in battles. There, a figure of 20% would be roughly accurate.