It's not overstating the case though. When a huge component of the system isn't open source, that clouds the openness of the system as a whole. People make fun of Stallman for his ridiculous practices, but at least he's principled about this very issue. Here, the AOSP maintainer is making the same principled stand. If you can't boot the device (hence use it) because it doesn't have open GPU devices drivers, then it's not an open device, plain and simple. None of the other stuff you allude to matters if you can't use the device.
As with many questions of openness, it's a question of pragmatism. It might be fair if some Kickstarted device did this or something, but this is Google's flagship tablet. Google uses the "Open" moniker as a marketing tool to both sell the devices to like-minded supporters as well as to bash others (Apple, MS, etc) pretty much whenever they can, as do many of their affiliates/partners. As such, it's completely fair to hold them accountable when that ends up not being the case.
Sorry, that's dumb. If it's really a question of "pragmatism", then in a practical sense there is no distinction between this device and, say, the HTC One which uses the same GPU and is subject to the same restrictions.
Yet I can go to www.cyanogenmod.com and pull and build a tree for the One and boot it, with access to the source code for virtually all of the system (sans things like the GPU blob in question).
That you would argue this is no more open than iOS or Windows is weird, and sounds more like a stand on vain principle and not "pragmatism".
If you can't even see the screen of the phone you're booting then how does it matter if the rest of the kernel is Free or not?
A lot of the theory of optimization can be reduced to speeding up parts of code actually being executed and paging out parts of the code never actually touched, which works because the source is not uniformly useful. A kernel would be useless without a scheduler, a GUI program would be useless without an event loop, no matter how "open" the code surrounding those are.
As with many questions of openness, it's a question of pragmatism. It might be fair if some Kickstarted device did this or something, but this is Google's flagship tablet. Google uses the "Open" moniker as a marketing tool to both sell the devices to like-minded supporters as well as to bash others (Apple, MS, etc) pretty much whenever they can, as do many of their affiliates/partners. As such, it's completely fair to hold them accountable when that ends up not being the case.