I would counter that and say, "An 'Open Standard' created and then implemented by 1 party is no different than a closed standard".
However, Khronos[1], the entity that makes opengl, also made the webgl standard. The fact that mozilla hasn't done as good of a job implementing it as google has doesn't magically negate the fact that it is still an open standard (that mozilla has actually implemented, just poorly).
Now, maybe there's a greater chance that the situation will change, and other suitable implementations will eventually arise. But until that happens, an open standard with one usable implementation really isn't different from a closed standard at all.