It was a bit of a tongue in cheek comment. It’s very unlikely they were even planning this when they bought GitHub. While you’re completely right about their rationale, a lot of people were at the time concerned about the acquisition, and it feels now a bit short sighted to burn all the good will they have generated by an act like this which undermines open source.
I don’t even think most people care about open source code. What if they’re reading private repos as well? The only option to escape this is to leave github. Which I’m kinda considering even with all the work involved.