If source repositories have been enabled initially and download_source_packages does it's thing it's not gonna work. Either forcibly disable (not enable) source repositories initially, or maybe not empty the package sacks but _getSacks(arch="src") and/or not forcibly enable enabled source repositories.
Fixed
Metadata Update from @kanarip: - Issue assigned to kanarip - Issue set to the milestone: 2.0.5 (EL-5)