Technically speaking, a license only has to require that changes be licensed under the same terms as the original to be deemed "copyleft".

The requirement to release source is specifically a GPLism and not part of all copyleft licenses. It's not infeasible for a copyleft license to imply "sure, it's legal for someone to create and distribute derivative works based on top of your modifications, but only if they have the technical skill to reverse-engineer it".