I agree, this line in the release notes is a bit vague. To give more details, the custom presets were not migrated automatically when a user was upgrading from PL4 to PL5.2 directly.
It you have used versions before PL5.2, you were not impacted by this bug.
I just upgraded to 5.3.1, and so I also looked at the current release notes (for Windows).
Interestingly, the release notes don’t mention version 5.2.1 any more. Removing details regarding an existing and published previous version from the release notes (or version history) is an absolute No-Go, IMHO.