I suspect Apple's interest in ZFS came from when they still made Xserve storage racks, and when they were looking at having to massively expand their datacentres in preparation for iTunes, Mobile Me, and the iPhone. They were maybe hoping to use OXS Server / ZFS there.
Since then, they've stopped making Xserve storage racks, and buy in their storage from other companies, and probably found that running largescale datacentres is HARD.
I strongly suspect that their datacentres don't run on OSX. Probably some form of massive unix with massive databases.
Now that they have a solution implemented, some of the internal drive for Apple to sort out ZFS seems to have evaporated. Plus, what they have seems to be working reasonably fine now.
Bringing ZFS up to the standard where they can use it in their data centres (and they will have to, if they're to be a creditable seller of ZFS) will involve a massive amount of work, and considerable risk in their having to disrupt a working solution and implement an untried system. And not very many new sales of OSX Server overall.
OSX Server, while suitable for running departments, and small to mid-sized companies simply does not seem like a large enterprise system to me. It seems to lack the back-end tools that Windows enterprise, and the various large-deployment unixes have.
Apart from Apple HQ itself, I cannot think of any other large company that runs its entire enterprise on OSX Server. I'm sure there must be one or two, but I can't think of any.