Also, I don’t see how this is true. Code on my website is publicly accessible, but not in the public domain, nor licensed for re-use, unless I say that it is.
Licensing matters for things that would be forbidden without permission by copyright law. Fair use is an exception to copyright law. Microsoft's explicit legal theory around Copilot is that ingesting code for it (and ingesting content to train ML models more generally) is fair use. If there theory is correct, license is irrelevant, there is no legal (at least copyright-based) barrier to them using any source code they can get their hands on to train Copilot.
Also, I don’t see how this is true. Code on my website is publicly accessible, but not in the public domain, nor licensed for re-use, unless I say that it is.