I suppose more concretely "doing something related to what you are writing about". On principle I have nothing against pure philosophers, who integrate old theories and formulate new ones. However, in my experience, writings like that of Marcus Aurelius are more compelling, who was a success in his own right before ever writing anything. Most modern "business" authors don't have any business (other than selling books to people). Taleb is just warning people to not take advise from authors who don't actually have "skin in the game".

De Botton does not write about things he cannot know about. He has not proposed a Grand Unified Theory of Physics or a mechanism for how matter becomes conscious. He writes about things everyone can relate to, which means he can relate to them as well -- but the work seems to be in thinking about things in non-obvious and novel ways and in the actual writing process. The quality of de Botton's writing really speaks for itself.

It must be difficult work in itself because even Taleb, when he took a stab at this 'pseudo-philosophy' genre with his book The Bed of Procrustes, managed to produce absolute piffle which would never have been published if Taleb weren't already famous for his popular finance books.

There seems to be an underlying presumption that writing is not real work and so people should be successful in the real world before they should feel entitled to write anything.

Couldn't agree more. There's something to be said for life experience which filters out the irrelevant and the non-important. I've always appreciated the story about the pit trader that earned millions in his career trading green lumber, all while assuming it's just lumber that's been painted green. It's too easy to theorize in an ivory tower and miss what's actually important.

It's elitism and it's a bad heuristic. There are lots of situations where someone might know a lot about X yet for some reason never did X. Being able to do X, and being able to usefully explain X are two different skills and few people can do both.