I wouldn't say "less competent". I'd go with "more risk averse".
The problem appears more clearly when you study the investing habits of women. By definition, mutual funds can't discriminate against Sandy in preference to Tony. So they'll both get the same rate of return on a given mutual fund. I'll stick to the US, but I imagine most of Europe is the same. Most people get their retirement plans through work. So Sandy and Tony are both given the same options, almost always a selection of mutual funds. The funds will be a mixture of stocks, bonds, and treasuries, with varying asset mixtures for both stocks and bonds. All will rate the riskiness of a given vehicle, but all are relatively safe as they have to pass some government tests to be offered. Despite having a shorter life expectancy and therefore probably less need for money after retirement, men prefer risky investments and therefore typically get a higher rate of return in spite of the occasional capital loss. Despite having a longer life expectancy and therefore a greater need for money after retirement, women typically invest in the less risky investments and wind up with a lower rate of return despite rarely loosing capital. Over a lifetime of investment, the divergence is frequently hundreds of thousands of dollars.
So it looks to me like the authors of the study correctly identified the primary component of the differential (Women were more likely to set a higher starting price on the online tat bazaar, pay to set a reserve price minimum, and opt for a "buy it now" option over a "best offer.") then blew right by it because it didn't meet their predetermined outcome for the study. I'd bet the next most important component is how the types of things the two groups sell breaks down. My mother would be likely to sell flowers and baked goods. My dad* would be more likely to sell car parts and appliances. Assuming both have the same sales skills, who do you thing will make more money?
*Full disclosure: My dad's first sales job was actually working as the delivery guy for a bakes goods company. He made pretty good money at it, almost always finishing as the top salesman in the company. After that he sold cars for a short time, but despised how shady the business model was, and he chose a "reputable" car dealer to work for. Then he sold appliances in preference to jewelry when he had a retail franchise. He finished his career selling HVAC installations.