I believe that the only metric used for this study was the ability to redeem miles on that specific airline's metal, and more specifically, the domestic coach award, which is is the goal of most frequent flyer members.
While I agree (from my own experience) that US is the stingiest of the legacy carriers when it comes to making award seats available on its own metal, I completely disagree that US is at the bottom of the barrel in overall award redemption opportunities. US miles are relatively easy to redeem for UA awards, but they are priceless when it comes to redeeming awards for premium travel on Star Alliance carriers. If the DM member goes into the conversation armed with several different dates and routing options for a partner award, it should net positive results.
The only real improvement which I would recommend for US is to offer *A upgrade awards like UA and CO do. UA is worse than US because of Starnet blocking, but CO does not do Starnet blocking, and they offer *A upgrade awards.
US may have its faults, but to imply that it is the worst airline for mileage redemption without even mentioning partner carriers is shoddy journalism, IMO.