They keep saying "study" but this isn't a study, it's a math paper. This is a "model." The "model" shows this and that and those things, not "the study" of which there wasn't.
And what the "model" shows is that instead of invisible mass we can't see causing gravity, it could be alternating layers of invisible positive and negative mass.
There's even less evidence for negative mass being possible than for many dark matter candidates that "only" have to be massive and non-interacting (or weakly) with photons. It's always cool to kick around some new possibilities but this seems pretty weak. Negative mass opens up a big old can of worm(holes) too.
L