For the layman, is this "discovery" legitimate? Does this mean dark matter is erroneous? What are the cosmological implications? Is somebody getting a nobel prize for this?
No, the dark matter is still dark, and was never missing, because we know it's there from its gravitational effects. We just can't see it (hence "dark") and don't know what it is, really. But whatever it is, it's definitely there. Well, actually there are some who say it's an illusion, but nobody says it's missing.
The missing (baryonic, i.e. ordinary) matter on the other hand, is matter that theory predicts should be somewhere but we didn't know where it was, i.e. it was missing. This research says that it's in intergalactic space, which everybody kind of thought but wasn't sure. Now, using their techniques of looking at the way frequencies from fast radio bursts spread out they can basically "see" it, so it's not missing anymore.
A Nobel is not likely for this work, because it doesn't change our understanding of the Universe in any significant way... but it's pretty cool, anyway, they deserve a toast, and "found the missing matter" will look really good on their resumés.
because it doesn't change our understanding of the Universe in any significant way
It’s a shame, really. This is a very significant piece of the puzzle. It may be incremental progress but it’s still important. Nevertheless, your assessment has reminded me of an Alpha Centauri quote I’m rather fond of:
There are two kinds of scientific progress: the methodical experimentation and categorization which gradually extend the boundaries of knowledge, and the revolutionary leap of genius which redefines and transcends those boundaries. Acknowledging our debt to the former, we yearn nonetheless for the latter.
-- Academician Prokhor Zakharov, "Address to the Faculty"
