Not really speculation, its mentioned in the article. Theres not really any free lunch here, vanilla postgres is optimized to perform well within the constrain

Not really speculation, its mentioned in the article. Theres not really any free lunch here, vanilla postgres is optimized to perform well within the constraints of a single scale-up host, minimizing storage usage as much as possible. These Aurora style systems instead take the assumption that storage is relatively cheap in a cloud environment, and likewise with any compute task that can be scaled out instead of up. So move as much as possible out of the scale-up instance to scale-out storage and compute. Additionally Google is claiming their system is better than Aurora because google has a distributed filesystem, where AWS only has block storage (tied to specific compute) and object storage (much worse performance).