According to the thread, it incorporates fan-written spells, which are not under the CC license unless the authors put them there. Which is a good idea now if you want to be safe, but wasn't an option when they were being written, so they are not currently licensed that way. (And only the original authors can do so, which means that a CC-BY-SA compliant Fan Grimoire would be a massive and thankless task — which is why I said "something like", not the Fan Grimoire itself.)