The source code is a trade secret, and it is not secret anymore, that's what has been stolen.

If take the Coca-Cola recipe from whatever safe it is stored in, I don't think anyone will have a problem saying that I stole the recipe, even if Coca-Cola still knows how to make it.

I agree. I think "illegally cloned" would be a more accurate headline while still having as much clickbait as the author intended.

They're probably using the music and film industry definition of stolen (meaning downloaded).

Copied should be a better term I think