Well, if you look at the article's source, you get this gem:
Chromebooks still remain a small portion of the total U.S. market for laptops and netbooks. The devices had about 4 percent to 5 percent share in the first quarter, though that was up from 1 percent to 2 percent in 2012, according to Mikako Kitagawa, an analyst at Gartner Inc.
So, if the laptop market was ~33m units in Q1, that puts Chromebooks at ~1.5m for the quarter, which is the first thing approaching an actual number I've seen on Chromebook sales. Not sure how that spreads out between Samsung, Acer, and HP.