Don't try macosx10.4Bundle target but rather the macosxBundle target in build.xml.
Both targets require lipo found in macOS tools, but if you target only Intel or ARM processor, you could only copy the Java version of your choice (if you want an Intel version, change 3 times [font=courier new]javaHome_macosx_arm64[/font] property by [font=courier new]javaHome_macosx_x86_64[/font]) and remove the the other calls to lipo used to build an Universal OpenJDK.
I never tried this tip under another system. If it doesn't work under Windows, remove or adapt the [font=courier new]exec[/font] tag which copies legal files with their symlinks). You could try also under to run Ant under Linux.

The Intel or ARM OpenJDK 15 must be installed in the folder indicated in the properties [font=courier new]javaHome_macosx_x86_64[/font] or [font=courier new]javaHome_macosx_arm64[/font].

Note that other targets like macosxInstaller and macosxSignedInstaller require also hdiutil and codesign which are tools of macOS.