Well Apple backs-up iMessages by default anyway. So even if it is end-to-end, they still have a copy of everything on their servers, unless you disable the iMessage syncing.
Also, just like Whatsapp's "end-to-end" crypto, just because they use it now doesn't mean they can't disable it at any time without you being aware of it. That's the danger of proprietary closed-source software.
Do you have a source for that? It would seem to be huge news.