In short, Apple maintain a 448 kB diff which they 'throw across the wall' in the form of an opaque tarball, shorn of all context. Many of the changes contained within look potentially security-related, but it's been released in a way which would require a huge amount of work to unpick.

That level of effort is unfeasible for a volunteer upstream developer, but is a nice juicy resource for a motivated attacker. Apple's behaviour, therefore, is going to be a net negative from a security point of view for all other users of this library.