stating the obvious. millions upon millions of devices in the world run iOS, of course the # of beta users would also be higher.
Percent doesn't really mean anything. What if the site had less overall visitors, and the percentages equaled the same number of beta devices?
Percentages could be indicative of something, but the article doesn't make it clear. In the second paragraph the article cites an increase in the percentage of "mobile devices" running iOS 7. That doesn't really tell us much. As you have said, that can mean a lot of things. However, the third paragraph cites an increase in hits from iOS 7 beta users compared to hits from iOS 6 beta on iPhones in particular. That does tell us something. Of course it only applies to this one site so I'm not sure if it's clear proof of a trend.