Hi there, and welcome to the forum!
I haven't heard this term before, more common are "trashcan", "by exclusion" and more...,
but can imagine several things it could mean:
Praps that they are still looking if there's anything else too, as the ACR 2016 criteria say it's not a diagnosis of exclusion.
Or that they are pretty sure it's that, but are looking testing for similar conditions and will possibly change the diagnosis then.
Or that the term "fibro" is a placeholder until we know and understand what conditions it actually consists of, is caused by etc.