Your logic involves quite a few jumps to conclusions that aren't remotely proven by the contents of the paper.
Although this does not support the "more qualified" argument, the study was specifically tailored to control for attrition from pregnancy, and the authors conclude that they are not entirely sure why older female executives quit at a higher rate than comparable male executives. It's not unreasonable to assume that it could be due to "discrimination" in the form of indignities and unfair assignment of tough/menial work, as the authors suggest.