"Donation of text is irrevocable" seems to be the only thing that implies a license from contributor to TV Tropes. It's pretty vague and I certainly wouldn't want to sue claiming a license with that to back it up.
It's also unclear whether that text existed before the switch to CC-SA-NC. Obviously it was updated, but was just the license changed? This is significant because contributing under CC-SA to someone using it commercially is possible. Contributing under CC-SA-NC to someone using it commercially doesn't make any sense, so they would need to have another license.
Good points. I don't know if the edit page had a mention about irrevocable relicensing back then though (the Wikipedia description seems to suggest that it wasn't the case).
It's also unclear whether that text existed before the switch to CC-SA-NC. Obviously it was updated, but was just the license changed? This is significant because contributing under CC-SA to someone using it commercially is possible. Contributing under CC-SA-NC to someone using it commercially doesn't make any sense, so they would need to have another license.