I concede that they don't have to release, that wasn't really my thinking. AC didn't say they had to release because of license, for all I know it may be some corporate charter or warm fuzzies that compels them. I genuinely don't know.
I guess I am trying to say I don't know why this is news. As far as I am aware, they have released all previous versions, like AC said. The page https://opensource.apple.com/ seems to imply such.