This is why Google is analogous to Comcast here. Neither can mess with the content or impact its origin. They are just part of the packet transmission system in between.
Hence this is a huge win. Previously, AMP was served off of Google servers where Google was the man in the middle aware of and even able to manipulate the content. The scripts were running on their origin, etc.
Now with this signed exchange tech, the contract is between browser and origin server. The Google cache is now super dumb, which is a big improvement from a privacy and security perspective.