Ira Longini is co-director of the Centre for Statistics and Quantitative Infectious Diseases at the University of Florida, and a WHO adviser, so rather than being an idiot I'd say they're fairly well informed on this stuff. That's not to say they're right of course, but not an idiot.
As others said, "could" covers a lot of various scenarios.
I mean sure, if we all somehow forgot viruses are a thing, it conceivably _could_ infect 2/3 of the world population, but with us being aware of it and using quarantine and just normal precautions, no way this thing infects 5 billion people, as it stands now.
In Wuhan there's 12 million people, even if the reported numbers were undershot twenty times, it would still have managed to infect "just" around a million people, while running unchecked since late November until mid-January.
I'm sure that person is trying to say something, I'm just not sure what.