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.