Fig. 3 lowest estimate for # of English speakers is Current population of the Anglosphere (Australia / Canada / New Zealand / United Kingdom / United States) = 435M; I'll eat my hat if What the hell was that estimator smoking? Hell, the max estimate there (~500M) is probably closer to a reasonable minimum.
The figures in the article are from 20 years ago. Back then, populations were smaller and many countries were just ramping-up English education for students.
Population of Anglosphere in 1992: 364M... <300M speakers still seems very low even 20 years ago.