I could see it being inferred that way but, the way I read it, they are not meant as unilateral facts. Rather, they serve as rhetorical examples of where you might find contractors doing similar work, but where the one more in service of "public good" is taxed higher because it's open source. Strictly speaking, Windows bits are not all closed source and there exist closed source Linux bits. But it's not a point that really matters in the context of the conversation.

I think it's fair to use Windows and Linux as stand-ins for closed vs open source because it's a very accessible example. And knowing the technicalities clearly doesn't undermine the argument.