Probaly alot of shorthand writing, anything with an R0 rating below 1 tends to die out. therefore you compare the sars rating of 1.49 and the novel corona virus of 3.8 by seeing how many times .49 fits in 2.8 which is around 5.7 times more infectious instead of 7.75 mentioned in his tweet (He probaly divided 3.8 by .49 instead of 2.8)

Ratings are just approximations and i'm no expert in virology to say wether this means anything at all.