The why is a much harder question.

You’re right about it probably being true, this is not the first study to find something similar, there’s two others reported on here: Patients have better outcomes with female surgeons, studies find

It’s interesting that this study looked at the proportion of women on the surgical team:

Overall, female surgeons performed 47,874 (6.7%) of the operations. Female anaesthesiologists treated patients in 192,144 (27%) of operations.

Hospitals with teams comprising more than 35% female surgeons and anaesthesiologists had better postoperative outcomes, the study found. Operations in such hospitals were associated with a 3% reduction in the odds of 90-day postoperative major morbidity in patients.

There’s some speculation in that first link about differences in aggression and risk-taking. But, given the relative rarity of female surgeons, it could just be a competency effect. If women are a very small minority for reasons not related to competency, and 93.3% of surgeons are men, it suggests that almost half the men are in the job because a more competent women didn’t get it. Groups with more women do better simply because they didn’t discount half the talent pool quite so heavily.