The author is arguing that the rate at which it was increasing (18x between 2008 and 2015) and the persistence of the trend (every year for the past three years) as well as that the most major contributing factor (tax law) is unlikely to change made this newsworthy, not the proportion of the population.

Edit: And I don't find it to be a moving counter argument. Additionally, I'm not sure its a good apples-to-apples comparison as the phenomenon seems to be prevalent among US ex-patriots, rather than the entire population.

Edit2: It turns out that it's hard to know how many americans live overseas[0], but it seems like 6.8 million might be a good guess. 4.2k / 6.8m is still only about .0006% though.

[0] http://www.migrationpolicy.org/article/counting-uncountable-...