It's capable of doing IO against a real ZFS array without any other code. ARC is an implementation detail and not necessary for correctness. If you removed ARC from ZoL it would still work, just slower. ARC is far from the most interesting milestone for a reimplementation effort because an ARC implementation doesn't need to be anything like the Sun version internally, as long as it offers similar performance.
This project is cool not because you're going to run the Python in your kernel today, but because someone can use it as a documented reference implementation of all of the data structures and transactions that is not covered by the CDDL, so another implementation based on this can live in the Linux kernel without problem.
If the GPLv2 GRUB ZFS code[1] wasn't enough to get someone started then I doubt this will make any different in porting ZFS to GPL given there would be more work involved in turning this into a usable kernel driver.
Not taking anything away from the work that the author has done though. It's a nice project. I just think a little pragmatism is needed before we get carried away with the ZFS GPL comments.
This project is cool not because you're going to run the Python in your kernel today, but because someone can use it as a documented reference implementation of all of the data structures and transactions that is not covered by the CDDL, so another implementation based on this can live in the Linux kernel without problem.