personally, I don't have interest in zsh because bash is the default shell in Linux, which is the UNIX system I use (I find mac's unix to be really a weird variant). Mac switching to zsh is just more evidence they're diverting from the mainstream.
It's closer to BSD than it is to Linux. Not super similar, but closer, especially in ways one tends to notice on the command line.
I've used freebsd for years and I still find Mac OS to be the least useful UNIX.
Further, unless I'm missing an important technical detail, Linux could, with some effort, pass the certification process.