> they can simply change the license terms to prohibit that use

GitHub's argument* is not that they're following the license but that the license does not apply to their use. So they would continue to ignore any provision that says they can't use the material for training.

Previously discussed: https://news.ycombinator.com/item?id=27740001

Moving off GitHub is a better step at a practical level. But again they claim the license doesn't matter, so even if it's hosted publicly elsewhere they would (presumably) maintain that they can still scoop it up. It just becomes more work, for them, to do so.

*Which is completely wrong in my opinion, for the record