I think you'll find that he's going to say they don't need a GPU but rather they help with their operation. While you can have your crew do a battery start as you climb on board your sweltering airplane for my money and comfort if for some reason I don't have an APU my choice would be a freon AC system and a GPU would be standard.
If a GPU is not available it's not uncommon to see Citation 650's and other airplanes crank an engine which burns only marginally more fuel at idle than an APU would (200lbs or so per hour) to run their air conditioning. As an owner I would highly encourage mandatory usage of a GPU for a non APU equipped airplane as many operators already do.
That said unless something has changed in the times since I last saw it I can think of one current production business jet that does in fact require airstairs to use. I'm sure there will be an option in the future but my first hand look-see in 2008 revealed no air stair on the Embraer Lineage. A purpose built business jet based on the Embraer 190 airframe.