What exactly do you consider radical about his position in this instance?

Radical might not be my choice of words, but he's got a certain type of ideological purity around free software that's definitely on the extreme end.

Here's a paraphrased version of what he's saying:

"GitHub [...] has done terrible harm to our community.

GitHub was so bad for free software, all along[.]"

Now, if you were to define "our community" as the open source community, this would be laughable - very few people would argue that open source software has been terribly harmed by GitHub (on the contrary, the rise of GitHub has seen many projects flourish). I don't necessarily think he means the wider open source community, though - he's referring to his own Stallman flavor of free software.

Sorry I said radical because we use the same word in french but the meaning is probably a bit different, I should have said perhaps extreme or very opinionated?

I wanted to say that he cares very deeply about free software and the whole philosophy of it, which can make him appear as "radical" to some people.

I hope I was able to clarify.

And, like Zed, considerable engineering prowess. Not like that matters or anything.

There's a difference between talented programming and good engineering, and _why is almost a perfect example of that. Engineering is more about discipline and maintainability than cleverness and artistic expression.