The public health scientists wrote in his first tweet that he's "never seen an actual virality coefficient outside of Twitter", i.e. he's admitting to not being a subject-matter expert. So getting corrected by someone who's done their research is not that surprising.

He had never seen one larger outside of Twitter. I think that’s different than how you just represented it.

It doesn't say "larger" in the tweet. Edit: or is "outside of Twitter" supposed to refer to a region in the space of possible coefficients that are past the values observed in the spread of viral information on Twitter?

He is an epidemiologist, you have to give the benefit of the doubt here that he has seen a virality coefficient before.

Probably trying to avert citing the official numbers that may have been classified.