A better thing to suggest is to use multiple forges, including GitHub, and mirror your projects across them. This way you will have exposure and options; you won't be as tied to any one forge.
"we keep publishing all our credit card numbers for anyone to see, why do people keep taking and using them? :("
If you don't want your code to be trained on, perhaps don't make it public in the first place. Even GPL is fine with this, you aren't required to put it on the internet, you just need to send the source code to the requester, e.g. via physical media as they did in Stallman's time.
That link you provided only points out GitHub has integrated "create pull request with Copilot" that you can't opt out of. Since anyone can create a pull request with any agent, and probably is, that's a pretty dated complaint.
Frankly not very compelling reasons to ditch the most popular forge if you value other people using/contributing to your project at all.