It was a systematic review, not a “singular ‘research study’”. A newer 2019 Cochrane review[0] found about the same:
> The evidence was limited in applicability and was incomplete. The included studies only presented data on gingivitis, plaque, and adverse events; most did not measure or report other important outcomes such as periodontitis or interproximal caries. … Additional use of floss or interdental brushes compared to toothbrushing alone may reduce gingivitis or plaque, or both, and interdental brushes may be more effective than floss. The evidence is low to very low‐certainty, and the effect sizes observed may not be clinically important.