I changed it to this, which I hope is clearer:

 This is a boolean value, or a string "True" or "False". If set to "True", the importer will remove any ISOs
 that are currently in the local Pulp repository that are not found in the manifest at ``feed_url``. If
 "False", old ISOs will not be removed. This parameter defaults to False.

I hesitate to say that the ISOs are from a previous sync, because if the ISO got there by being copied or uploaded, it will still be removed.