The advice is sound because it circumvents a hazard of trying to become a 'generalist' - Most make the mistake of thinking the formula for Renaissance Man is 'learn everything'. I think it's more accurately to spend 60-80% of your time going deep on 2-4 specializations (the more diverse the fewer) while spending 40-20% of the time getting a broad overview of anything else that interests you. Do for x years, choose new specializations, rinse & repeat.