According to the contest rules, the “winner” is just the argument with the highest mean donation, if it statistically beats the control. It didn’t have to statistically beat the other arguments, and as you note it did not do so in this case.
But many won’t interpret it that way and further clarification would have been good, yes.
Eric Schwitzgebel responded as follows to a similar comment on his wall:
But many won’t interpret it that way and further clarification would have been good, yes.
Edit: Schwitzgebel’s post actually had another title: “Contest Winner! A Philosophical Argument That Effectively Convinces Research Participants to Donate to Charity”