He was wrong anyways. I actually just figured it out. So for anyone else wondering.. settings..accounts and passwords..iCloud...turn off safari.
He may have been wrong but he was volunteering his time to help you.
Yes, what you discovered is that you're sharing browsing history with another device using the same Apple ID, as I'd suggested. Turning it off solves the immediate problem. However, it's still a good idea to find out who you're sharing an Apple ID with.