"Open Source" means something specific, but I disagree that applies to all usages of the word "open" (or even within computing), especially when those usages predated Open Source.
My point is that while it doesn't apply in this case, it's often misinterpreted as such; in other words, people see the word "open", they see that it's available on Linux, and they make the obvious deduction from that. They can't be blamed for making that deduction because it is the obvious deduction in today's software landscape, despite the fact it it wouldn't have been when OpenGL was first specified.
So in other words, and entirely unintentionally, the word "open" ends up misrepresenting OpenGL as something that it isn't, and it's desirable to remove that misrepresentation.
I am aware that if "open" was removed from NG then there would be choruses of "the old OpenGL was better because it was open source and open source is always better but this one isn't open source so it's worse", even though the old OpenGL wasn't open source (and wasn't even software), and that's all part of an incorrect perception of what OpenGL is.