Earlier quoted context omitted.
That comment is replying to someone saying "Windows is a rootkit". To me it looks like someone replying to an absurd, untrue statement about Windows with an obviously untrue, absurd statement about Linux. Unless this is part of a larger pattern of inaccurately describing Linux, I don't believe he literally believes what he is saying in this comment.
I don't know, doesn't read like a joke exaggeration to me. I think the most likely thing is that he's confused about firmware blobs and is mixing things up. He probably never looked much at Linux and just heard a few things here and there. He was at Microsoft during the Balmer "Linux is cancer" and "get the facts" era, so that probably coloured his view a bit. That said, I agree you shouldn't attach too much meaning…
But generally it seems like he uses all three of Linux, Windows, and Mac and talks positively about each of the three.