Both Boeing and Airbus make good airplanes. Nevertheless, they hire expensive salespeople and pay them massive commissions. They don't just tell the airlines to download a pdf brochure of the planes' specifications and call them when they're ready to place an order.
People making statements such as "good products sell themselves" are probably thinking about gadgets like iPhones that don't require "greasy used car salesmen" with high-pressure tactics to close a sale. Yes, the Apple employees at stores are more like order takers than salespeople.
However, many well-engineered products absolutely require salespeople to close the deal. I was recently researching $100k CNC milling machines -- there are several brands with quality engineering and yet they all have salespeople to guide prospective customers about configurations and persuade me to buy. Trying to decide purely based on information posted at the manufacturers website and printed brochures would be unwise. Yes, the general "concept of a CNC machine" sells itself but a particular brand & particular model of CNC does not.