iCloud Drive issue between High Sierra and El Capitan

Hello,

I have just added an older laptop into my ecosystem.

It's a MacBook Pro (15-inch, Mid 2009) running El Capitan (10.11.6).

It's the latest version of OS X / MacOS that I can run.

All other systems are running MacOS High Sierra and iOS 11.

iCloud Drive doesn't seem to have a sync function for apps that synchronise using it.

For example, these do no sync:

- Ulysses documents

- Airmail 3 account settings

I have checked iCloud settings > Options > App selected (to ensure documents use iCloud)

I have signed out > rebooted > signed in again (annoying as it's re-downloading a 200GB Photo Library)

What am I missing?