That was a very strange sentence to read. Is that really how little they assume the reader knows? I almost took it as a comment on the expected quality of the peer review.
It's also worth noting that for a paper from the collaboration to have to got to the preprint stage it will have gone through a very strict internal review process with several layers.
Source: I working in the LHCb collaboration for several years.
tl;dr: It's a tetraquark, not a fundamental particle. We've seen tetraquarks before, just not one made out of four charmed particles. And it's not 100% clear that it's actually a tetraquark rather than two mesons.
