Earlier quoted context omitted.
On one occasion the traffic to google cloud is systematically not proxied: every time one sends an attachment in iMessage, the file (or the media) is encrypted on device and sent to gcs-{eu,us,asia}-00002(?).content-storage-upload.googleapis.com, received from gcs-{eu,us,asia}-00002(?).content-storage-download.googleapis.com This should be pretty visible to Google, the rest of the traffic is handled better.
So the bucket is open to the world to write
It just means Google may provide access to metadata outside of Apple’s control. Those metadata could be useful to do classification of anomalies on the basis of pattern of life analysis, or similar.