Earlier quoted context omitted.
The rule is like: It's really really hard to make startups without the open-source Unix-based systems/tools
The point is that the phrase "exception that proves the rule" should only be used when talking about man-made rules and laws, not inductive hypotheses. You don't get to use evidence against your hypothesis as evidence for your hypothesis! Put another way, when used correctly, "the exception that proves the rule" is a valid rhetorical phrase regardless of how many exceptions there are. Whereas if there were 500 succes…
But you do. That's the point. It's not an absolute law. I'm sure you can find a great application written in Visual Basic. This doesn't make Visual Basic a great choice over Python.
If you could list 50+ successful startups running on Windows, then you could say there's a strong evidence.
And even then, this only proves that Windows is as good as Linux. It doesn't counter the original point about Linux being great despite not having a business model centered around IP.