The fact that a lot of Chromebooks are being sold is fairly solid; I would reckon that the questionable claim is that Chromebooks have strong sales traction with retail consumers. I bought myself a Lenovo Duet tablet last fall because I wanted a device that could last multiple days with intermittent streaming use and I missed the feeling of a cramped 10" netbook. I got it on sale for $200 USD and was floored by how nice the experience was on a PC that cheap. But friends and family who saw me with the tablet were shocked that I, an adult who likes computers, owned a Chromebook. I only know one other person IRL with a personal chromebook, and they bought it after being given a school-deployed one in college.

Is it mainly students in schools? They wouldn't have much reason to access govt sites yet

My anecdote: A lot of people seem to be using Chromebooks to replace tablets (because good, non-Amazon tablets that aren't expensive are rare these days). You can pick up a $250 touchscreen Chromebook that works as a vastly superior web-browsing and video-watching device than a $250 Samsung tablet.

Keep in mind that the majority of all fed gov visits are USPS tracking pages and the like. Chromebooks are majority used by students so I would expect this.

Every Chromebook I've seen in the wild is being used by a student or teacher. I suspect students don't have many reasons to visit government websites.

My whole local library is full of Chromeboxes. Maybe things like that make an impact.