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.