They are referring to AgileBits customers on the Mac App Store. So anyone who previously purchased 1Password on the Mac App Store. Apple doesn't offer upgrade pricing, so it would make sense that the upgrade is free. Most likely AgileBits calls that fact out due to past issues with the iOS App Store where they took down the old app and created a new one so they could charge an upgrade fee.

> "App Store customer [who have already bought a copy] get 1Password 4 for free"