It seems like it can be construed as a cash handout to owners of BTC, not a spinoff. At least there seems to be disagreement elsewhere on this thread, and from Levine's article.
I don't think refusing to require shorts to deliver BCH opens the door to any other complications. In fact, I think it's the simplest interpretation. You borrowed one BTC, or share, or pony, or whatever, and later you have to return one BTC. If someone else starts a new currency, fine, but that's nothing to do with your borrowing agreement. There's no mention of the price of anything; for all this simple agreement knows, BTC is the only asset in the world. (Though in practice I'm sure you'd have to post margin. Maybe some ponies?)
For shorting stock, the return of dividends and spinoffs is, I believe, a consensus agreed upon by the market as more closely reflecting what people would want - you can start a different stock-lending market that doesn't do this, but there doesn't seem to be much demand for it. To draw a clean analogy between BTC and an equity spinoff, you have to suppose this kind of agreement exists and the contracts signed. I'm not sure what it is that BTC shorts agreed upon, and it seems there isn't widespread agreement, but to me, the simplest and narrowest interpretation of a short, in case of any confusion, is "I borrow 1 BTC, I must return 1 BTC."
(Again, I have no BTC experience, just finance experience, so if someone out there really is short BTC and has wrangled with this stuff, I'd be happy to hear from them.)