https://gs.statcounter.com/os-market-share/desktop/north-america/#monthly-200901-202607
Cloudflare stats also seem to support peaks of 15% usage but around 9-10% on average.
The trendline suggests this is just the start.
"Looking at browser headers" usually means they're parsing the User-Agent string, which all kind of people (and bots) spoof for all kinds of reasons. There even used to (and might still) be a Firefox extension that would generate random ones.