> If you, not I, uploaded my GPL'ed code to Github is the blame on you then?

Definitely not me - if your code is GPL'ed, then I'm legally free to upload it to Github, and to an extent even ethically - I am exercising one of my software freedoms.

(Note that even TFA recognizes this and admits it's making an ethical plea, not a legal one.)

Github using that code to train Copilot is potentially questionable. Github distributing Copilot (or access to it) is a contested issue. Copilot spitting out significant parts of GPL-ed code without attaching the license, or otherwise meeting the license conditions, is a potential problem. You incorporating that code into software you distribute is a clear-cut GPL violation.

The GitHub terms of service state that you must give certain rights to your code. If you didn't have those rights, but they use them anyway, whose fault is that?