I wouldn't even call it "promising but inconclusive" so much as "not conclusively a dead-end for further research". In a single-arm open-label study, with no blinding, both the participants and the researchers know who's getting what. You need a placebo and double-blinding for comparison against the active group and to adjust for any ways in which the researchers may have unwittingly influenced the results. (Or perhaps even wittingly, when there are conflicts of interest. I spent half a minute looking up this study and didn't see any statement attesting that there were none.)