protecting their content by licensing it explicitly.
You can do whatever you want, of course. But any license you put on your content here protects it less than not putting any license at all. That's after all what licenses are for, granting people use of your content.
So you're not so much protecting your comments, but graciously allowing them to be used for training for non-commercial purposes, where most people are greedily keeping them to themselves. I suppose that's admirable.