I don't think the spec is clear at the moment, but I can't see it not also applying to communities -- write the post, choose to post it in a community, schedule it to post to that comm.

(And to answer your followup -- scheduled posts will be available to everyone, just at a higher limit for paid users. So, like, maybe free users will be able to have up to 3 scheduled posts at any given time, while paid users might have 10 and premium-paid might have 20. Or whatever. *is making numbers up completely out of nowhere*)