Sign flat package with non-Apple certificate
Yes, it is so. The certificate must be a Developer ID Installer certificate from Apple. You cannot use the same code signing certificate you have been using for your Windows application previously.

You can read the instructions on how to properly sign flat installer packages here:

https://developer.apple.com/library/content/documentation/IDEs/Conceptual/AppDistributionGuide/DistributingApplicationsOutside/DistributingApplicationsOutside.html