If you are selling it on then you also need to make sure that you log it out of the iCloud so that they can login. I didn’t look at JPs link but I read that this is a separate setting to Reset.
That link is just a ad for companies to charge for a unneeded service. All you have to do is log out of iCloud on the device. Which what I said above will do.
“Before giving away or selling your iOS device, be sure to erase your content and settings (in Settings > General > Reset > Erase All Content and Settings). When you erase your content, Find My iPhone and Activation Lock are also turned off.”