The ant dependency is only used for the excelant tasks [1] (which are never used in Debian [2]). Any use of these tasks implicitly means Ant is already on the classpath. In this context Ant can be seen as a runtime, much like the Servlet API for web based stuff. So I think it makes sense to remove the dependency from the binary package.
Splitting ant into ant+libant-java is a good idea, but this can probably wait for the Buster development cycle. Emmanuel Bourg [1] https://poi.apache.org/spreadsheet/excelant.html [2] https://codesearch.debian.net/search?q=excelant