I'm not sure if this counterpoint generalizes entirely to the original critique, since certainly LessWrongers aren't usually posting about or discussing math as if they've discovered it-- usually substantially more niche topics.
Like mathematical logic (in the intersection of math and philosophy) didn't have that many true predecessors and was developed very far by maybe only 5-10 individuals cumulatively, or information theory was basically established by Claude Shannon and maybe two other guys, or various aspects of convex optimization or Fourier analysis were only developed in the 80s or so, it stands to reason that the AI-related applications of various aspects of philosophy are ripe to be developed now. (By contrast, we don't see, as much, people on LW trying to redo linear algebra from the ground up, nor more "mature" aspects of philosophy.)
(If anything, I think it's more feasible than ever before, also, for a bunch of relative amateurs to non-professionally make real intellectual contributions, like noticeably moreso than 100 or even 20 years ago. That's what increasing the baseline levels of education/wealth/exposure to information was intended to achieve, on some level, isn't it?)
The problem is less clear in philosophy than mathematics, but it's still there. It's really easy on your own terms to come up with some idea that the collective intelligence has already revealed to be fatally flawed in some undeniable manner, or at the very least, has very powerful arguments against it that an individual may never consider. The ideas that have survived decades, centuries, and even millenia against the collective weight of humanity assaulting them are going to have a certain character that "something someone came up with last week" will lack.
(That said I am quite heterodox in one way, which is that I'm not a big believer in reading primary sources, at least routinely. Personally I think that a lot of the primary sources noticeably lack the refinement and polish added as humanity chews it over and processes it and I prefer mostly pulling from the result of the process, and not from the one person who happened to introduce a particular idea. Such a source may be interesting for other reasons, but not in my opinion for philosophy.)