It’s not to me either, but you’re missing the point. I think most of graphic cards today are capable of handling the zen garden easily and there’s no need for involvement of the CPU. Metal makes a big difference only on mobile, and while it’s good to have a similar tool on android as well, but I don’t see that happening any time soon considering vast range of CPUs used on these devices. So in the end, it makes sense to have it on iOS only, at least for a good while.