I think you could approximately define philosophy as "the set of problems that are left over after you take all the problems that can be formally studied using known methods and put them into their own fields." Once a problem becomes well-understood, it ceases to be considered philosophy. For example, logic, physics, and (more recently) neuroscience used to be philosophy, but now they're not, because we know how to formally study them.

So I believe Wei Dai is right that philosophy is exceptionally difficult—and this is true almost by definition, because if we know how to make progress on a problem, then we don't call it "philosophy".

For example, I don't think it makes sense to say that philosophy of science is a type of science, because it exists outside of science. Philosophy of science is about laying the foundations of science, and you can't do that using science itself.

I think the most important philosophical problems with respect to AI are ethics and metaethics because those are essential for deciding what an ASI should do, but I don't think we have a good enough understanding of ethics/metaethics to know how to get meaningful work on them out of AI assistants.