How is it known when the quote you gave says "may be" which implies it's not known and is speculation on their part as well?
The reply to the "it may be..." message is from a @sun.com email address, and confirms "that is the essence of it".
Fixed links for the message and reply:
https://web.archive.org/web/20091028/http://mail.opensolaris...
https://web.archive.org/web/20091028/http://mail.opensolaris...
There is no reason to speculate as the reason is know (as stated by Jeff Bonwick, one of the co-inventors of ZFS):
>> Apple can currently just take the ZFS CDDL code and incorporate it (like they did with DTrace), but it may be that they wanted a "private license" from Sun (with appropriate technical support and indemnification), and the two entities couldn't come to mutually agreeable terms.
> I cannot disclose details, but that is the essence of it.
* https://archive.is/http://mail.opensolaris.org/pipermail/zfs...
* https://web.archive.org/web/*/http://mail.opensolaris.org/pi...