iCloud: Safari Syncing

You wrote - Something I was wondering – I know that to sync between two MacBooks, you need to be logged into the same iCloud account on both. But do both computers need to have Safari syncing enabled in the iCloud list?

Both Macs must be signed in with same apple id and password , with latest update and connected to high network speed .Yes if you open system preferences > iCloud > the box of safari must be checked in both Macs .

In the last about two Mac A and Mac B - if you uncheck the box of safari in Mac book B a pop window will appear so again check the box of safari and click on ok , the merging of bookmarks will start on both devices .Read this article Get help using iCloud Bookmarks and Reading List - Apple Support

In Mac B the bookmarks will not sync as the safari box is unchecked , as syncing of bookmarks is from iCloud server , and the box safari is a medium , kind of restriction ( check the boxes of safari they will start syncing and you uncheck the box of safari they will stop temporarily , may be you perform this step from any device A or B ) .

On the list of devices on the iCloud account, it shows that his MacBook is logged in – but I don't recall seeing any of his bookmarks, notes, safari history, reminders etc. on my MacBook.

See this article Check your Apple ID device list to see where you're signed in - Apple Support

Does that mean my old colleague has disabled the syncing of these things from his MacBook? And with syncig enabled on my MacBook but disabled on his, would my personal data have synced to his MacBook.

When your friend has signed out from Apple ID and password from his Mac the syncing will stop , and in the past if syncing of two Macs were performed the iCloud Keychain password are also shared see this article Copy keychains to another Mac - Apple Support

Note: Passwords stored in your iCloud keychain are encrypted and stored on your computer and can’t be copied. To transfer these keychain items to another computer, set up iCloud keychain on the other computer using your iCloud user name and password. The other computer will sync with the iCloud keychain items on your Mac.

It depends while syncing the iCloud Keychain box was checked or not , if in any case it was checked the passwords have shared , also the reminders , photos , calendars , contacts are shared in the process of syncing , when syncing stops ( the person sign out from Apple ID with password ) it stops .

Now , your friend has been far away it depends upon you to change the apple id or the password for old apple id used in syncing see some articles Change your Apple ID - Apple Support

Change your Apple ID password - Apple Support