Yes, this is correct.
If the input document provided is encrypted with either the user and owner password or just the user password alone, then the document cannot be loaded without the user password. In such cases, IsEncrypted will return true indicating that the document is fully encrypted.
If the input document is encrypted with only the owner's password, and it can be loaded without any password, then the document is still considered encrypted and IsEncrypted will also return true
To determine whether the input document is encrypted with a user or owner password, please refer to the table provided in the documentation link below:
https://help.syncfusion.com/file-formats/pdf/working-with-security#how-to-determine-whether-the-pdf-document-is-protected-by-user-or-owner-password