There is a big difference between something being 'the best group ever' and being 'an easier shortcut to rationality than digging through philosophical writings the old-fashioned way', which is how I interpreted this post. There is a community component to LessWrong that obviously isn't present in old books

Reading books never was a good way to learn rationality. You need to learn it in practice, through discussion and debate, and you can do that in the context of mainstream philosophy because mainstream philosophy has its blogs and NGs too. (of course it doesn't have a "community" with a leader, a set off canonical works and a number of not-very provable doctrines everyone is supposed to subscribe to -- and it's better for it).