Sussman is not just "a professor at MIT." He's one of the inventors of Scheme and one of the authors of SICP. And perhaps most apropos for this article, one of the founders of the FSF, along with of course RMS.
Your comment could be taken out of context here. I am no language expert, but it's better to use the form "Sussman is not only a professor at MIT, but also ...".
"just" is probably overused these days. It has replaced words like "please" and structures like above.
Contrast:
a) Can you just do that
b) Can you please get that
You're the only person who said 'just'! You're arguing against something you said.