Technically, they're wrong because the stats are collected by trackers in sites about WebGL, so there's a bias towards having complete WebGL implementations.
Of course there are more than 1% browsers that support, but we're talking about implementations, not devices. You can can have an old iPhone running iOS pre8 and it won't run WebGL, but one can say that iOS does support WebGL now.
So firefox is a class, and there are instances of that class running. Most instances are running on mobile devices, so it really makes sense to say that only a small minority of browsers are capable of running WebGL.
I'd say most of mobile browsers can run WebGL, it's just that the experiences are not designed for mobile platforms (this Polygon Shredder, for instance, it does work on mobile, it's just too slow to be good, but I can probably make it work)
Of course there are more than 1% browsers that support, but we're talking about implementations, not devices. You can can have an old iPhone running iOS pre8 and it won't run WebGL, but one can say that iOS does support WebGL now.
The actual implementation stats are here http://caniuse.com/#search=webgl