Does he suggest software must be free?
We know he doesn't say the must be free in cost. Does he say they need to be free as in speech? I've never heard him say that. I don't think of him as telling others what they must do.
I feel like a lot of the polarization around what he says comes from misunderstanding. This feels like one of them.
Yes.
If you mean must as in, it should be illegal to do otherwise, I'm not sure. I don't think so, but given that he casts the conversation in terms of human rights, I can't entirely tell.
But if you mean must as in, does he really think all software, without exception, should be free? Absolutely. I think that's clear from the linked article alone.
I think the difference between must and should is significant.
Then think of him as telling others that they mustn't limit others
I would imagine they would stay safely (maybe even years) behind the cutting edge. But common availability of the source code would make it easier to share information that may be able to speed up acceptance testing or flag up issues.
You could argue it's actually more dangerous for an airport to rely on software that is opaque, for all they know there is some edge condition in there by accident (or on purpose) that could cause mayhem.
I am going to assume however that ATC software source code is rigoursly audited by a trusted partner of the airports using it.