According to Apple’s documentation Activation lock is enabled when you turn on Find My Mac and disabled when turned off.
https://support.apple.com/en-us/HT208987
How did it get activated in the first place if it the author never set up “Find My”?
That might explain why Apple won’t deactivate it. They don’t have a way of knowing if the OP sold it to someone and “stole” it back.
If the guy who got is legit OP may want to reach back out to that guy and ask him if that’s the case the guy who had it may be able to disable it from Find My.