Makefile attempts to execute GenerateModuleFiles.php which does not exist in this repo or in https://github.com/SynologyOpenSource/pkgscripts-ng . The website page https://help.synology.com/developer-guide/getting_started/first_package.html is what asks users to use this ExamplePackage in the first place, but then on that page it uses a different Makefile (without telling the user that it is different and they should modify their local version).
Likewise the create_package_tgz() in SynoBuildConf/install differs from what is on that page, and as it is, the SynoBuildConf/install is invalid, throwing:
make: *** No rule to make target 'packageinstall'. Stop.