Ah. The link answers to this question under "Advanced Data Protection and iCloud.com web access"
> When a user first turns on Advanced Data Protection, web access to their data at iCloud.com is automatically turned off. This is because iCloud web servers no longer have access to the keys required to decrypt and display the user’s data. The user can choose to turn on web access again, and use the participation of their trusted device to access their encrypted iCloud data on the web.
Then they explain that if you turn it on again, your devices will send your keys to Apple's servers for the duration of the web session. Technically this leaks your keys to Apple forever, but they promise that they keep it for the duration of the session.
> When a user first turns on Advanced Data Protection, web access to their data at iCloud.com is automatically turned off. This is because iCloud web servers no longer have access to the keys required to decrypt and display the user’s data. The user can choose to turn on web access again, and use the participation of their trusted device to access their encrypted iCloud data on the web.
Then they explain that if you turn it on again, your devices will send your keys to Apple's servers for the duration of the web session. Technically this leaks your keys to Apple forever, but they promise that they keep it for the duration of the session.