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 to a singular comment like this. Maybe he was joking after all, or maybe he was just wrong. We're all wrong sometimes, and that's okay.

Edit: he seems to have an entire "Linux vs Windows Round 1: Open Source vs Proprietary" video, but I didn't watch it: https://www.youtube.com/watch?v=a9ZADRy5W0c