They made some custom and probably more convinient way to backup using adb backup with tar files. The problem is that it is really hard to convert any backup (titanium or ultimate) for it to accept. I don't think it is bug, it is just some strange usage of adb backup. This means even if Helium creates .ab backup really it is not in that format... Or it is just unencrypted or uncompressed. And I don't have openssl and cygwin to confirm it.
---------- Post added at 01:20 AM ---------- Previous post was at 01:17 AM ----------
Yes. It looks like .ab created by Helium is just tar with ANDROID BACKUP header. Not encrypted, not compressed. Is it possible to create pack/unpack for such format?