I'm still a confused off by your comment "I have no idea what they have ever written at all. Robert Martin, .. and so on." when you say that you've "seen FitNesse in the past". Did you not know that he is part of the project?
Have you looked to see what projects they've been on?
FWIW, I don't think Fitnesse is well-written. When I looked at it about 8 years back there were several security holes in it, including no protection against directory traversal attacks.
It was clear that the developers had no experience working in an untrusted environment. Which, to be fair, is the security model for a FitNesse deployment.
(FitNesse.org is served by FitNesse-v20160618, so that's not 100% true.)
I don't trust Martin's views. I think he's full of bluster, and makes assertions far beyond what is justifiable. For example, he did a Prime Numbers kata some years back where it was clear he was gaming the problem - unknowingly, I think - so that his cute but inefficient two-liner would be an acceptable answer, while poo-pooing those who immediately thought to use a sieve variant.
I listen to Torvalds when he talks about how to write his software, because his software is a massive success.
I'd listen to NASA because their missions were a success.
Why should I listen to other people like Robert Martin? I had seen FitNesse in the past, and while I'm sure it's a useful and well-written bit of software, I don't anyone would remotely describe it a particularly notable success. Why would I listen to his ideas more than anyone else who's written a testing framework that some people use?