ELI5 - everything about a phone only works when the carrier allows it to connect to the network. So, the way this is done is to, well, connect to the network and allow the phone to be unlocked - which according to the article is what happened? So the real complaint is that the Pixel can't be loaded with a customized Android OS (or Linux, etc.) without being connected to the internet first and this is bad because the…
Because the vendor can anytime say 'fuck you' and disable/remove the process which allows unlocking the bootloader.
Edit: already happened with Pixel 2: https://news.ycombinator.com/item?id=35854552