That's true but it's hard to call OpenGL itself open-source. That's just an open-source implementation.
But the "open" in OpenGL really only means that the standard is an open standard, not that its complete implementation has to be open source (which, however, doesn't mean that open source implementations aren't allowed and don't exist... Mesa being one example).
Now, given the fact that all IHVs basically must supply a working OpenGL implementation for at least a couple of years to come (personally, I'd guess two decades) to cater both existing software and existing developers who are unwilling to migrate to a different API (quite possibly, Vulkan will not be an API for everybody!), it makes sense to provide a single open-source implementation (maybe a major IHV joint venture) which covers the current state of the art, or most of it.
At that point, when the open standard also has an "official" open source implementation, I find it legitimate to call it "open source", too. Nothing funny about it, if you ask me.
Once you have that single working implementation, all you need to support is your platform-specific Vulkan layer. As long as this works, everything else works, too, and the open source community can (and will happily) take care of maintaining the other beast.
This is both cost-effective, and enduser-friendly.