Factory reset

I did a factory reset on my phone before selling it. When the buyer tried to set it up, he said the activation lock is preventing him from using the phone.

I removed the phone from “Find My” but later read online that I should have selected “erase” first.

I am worried somehow my data remains on the phone, even though it is no longer linked to my Apple ID and I did a factory reset - can someone calm my panic? I don’t get why the activation lock was still in effect, even after a factory reset.