This is not a bug, just a feature that does not exist.
Since apparently nobody needed that in 10 years, I doubt it's of critical importance to the entire user base. Also, there's a trivial workaround in just using the svn command line in a shell script during the build to fetch externals that are otherwise ignored. Therefore reducing priority.
Please also take the time to read https://wiki.jenkins-ci.org/display/JENKINS/How+to+report+an+issue on how to use this tracker, otherwise nobody may see your wrongly filed issues.