The full article is not accessible by any means i'm familiar with, but the abstract suggests that it's, for lack of a better word, a virtual phenomenon.
Basically, they poked at Bose-Einstein condensate, and got it to behave as if it had negative mass.
What it reminds me of is negative electrical resistance found in things like gas discharges and some active systems.
While not physically possible in general, it can happen within certain regions - it's not literally a thing having a negative resistance, but a system behaving as if it had one.
TL;DR: Might be something curious, might be boring - no way to tell without more details (and maybe much more research).