I think his example was rather poor as I can think of at least 3 use cases off of the top of my head where having separate categories would be a good idea, and

I think his example was rather poor as I can think of at least 3 use cases off of the top of my head where having separate categories would be a good idea, and having varchar arrays would be a giant PITA. That said I'm not sure he was trying to make the point that upfront normalisation is bad in itself, just that doing too much "what if maybe someday we might want to separate this" kind of normalisation is a waste of time.