I'm not sure what point you intend, since Factor has never been popular. No offense intended to the authors and no shade intended, but I think that's pretty clearly the case.
There are other not-popular languages that build up from scratch. I'd hesitate to go too crazy in assigning causality to this particular characteristic. Being stuck to an existing ecosystem has disadvantages too. But it's not a counterexample to the statement you quoted.
What I am saying is that Factor has had an extensive ecosystem despite it never catching on. I cannot imagine how many libraries there would be were it had a vibrant community and ecosystem. Sometimes I do ponder.
In any case, "Factor has such an extensive set of libraries from just ~4 active developers." is nuts, in a good way.
There are other not-popular languages that build up from scratch. I'd hesitate to go too crazy in assigning causality to this particular characteristic. Being stuck to an existing ecosystem has disadvantages too. But it's not a counterexample to the statement you quoted.