wow, I was expecting an intelligent list of reasons why he cannot explain how mathematica works internally (I'm sure there are many). Instead, this post is a ridiculous strawman argument against the open development model.
For all the reasons he says, he could perfectly publish the source code of mathematica (e.g. under the AGPL) but still do not accept external pull requests. There are many successful open-source/closed-development projects out there.
I think you misinterpreted his point. It's not that he can't give Mathematica away as it stands now. His point is more that Mathematica couldn't be developed as a free and open project with just contributions from random people.
The point he doesn't mention is that it costs money to develop it closed source, and giving away the source (or even the binary) would cut off funding. I'm obliged to agree with him, since there isn't anything free that comes close to Mathematica.
For all the reasons he says, he could perfectly publish the source code of mathematica (e.g. under the AGPL) but still do not accept external pull requests. There are many successful open-source/closed-development projects out there.