I encourage you to read the article, as this is one of the things that I cover (including a quote from the developer of ClockwordMod regarding the practicality of using fastboot oem unlock for purposes of getting root on a device without anything to start with, such as kernel source in specific).
So, it is possible, it just isn't practical, especially when an exploit happened to be so easy to come by. Once you then have a security exploit, you can then start to ask "what does this let me do that an unlocked bootloader does not", and the ramifications on Glass are, at least to me, interesting.
Unlocking the bootloader just allows you to have access to boot an unsigned system image. He points out in the article that you'd still need an image to boot.
With an unlocked bootloader, you could boot something that then messed with the filesystem to drop "su" out in the bin (and this is what happens when you boot a recovery image to root an Android phone).
But in order to do that, you'd need kernel sources, so that you could have something that would properly boot on the device and mount the filesystem.
So, it is possible, it just isn't practical, especially when an exploit happened to be so easy to come by. Once you then have a security exploit, you can then start to ask "what does this let me do that an unlocked bootloader does not", and the ramifications on Glass are, at least to me, interesting.