It’s called “a joke.”
For clarification, the joke is that a “professional amp builder” estimates a 4+ year wait time on a guitar amplifier. It is a joke because that time frame to build a guitar amp is insane. Is building amps your 4th job or something? Do you personally pick out the cows that will sire the new calf whose leather will eventually cover each amp, every time an order is made? Do you personally walk around the untamed wilds of the world looking for new veins of metal ore like a prospector from the Old West, and upon finding them, do you setup a brand new mine and all associated extraction infrastructure to produce what will eventually become the refined metal to form the chassis you personally shape with a ball peen hammer and transformers that you personally wind with your bare hands? Do you forget how to make tube amps after every single one you ship and have to relearn circuit design from the ground up after each order? Do you have absolutely zero parts on hand at your shop and have to procure each component individually, one at a time, every time you start a new build? Is your shop 400 miles away from where you live and you commute to and from work every day?
Skyscrapers go from conception to doors open in less time. The world’s political climate changes in less time. Wars are fought and won in less time. A 14 year old middle-schooler will go to University in less time. A person can start and end a music career in (much) less time.
Either your build process is wholly inefficient to the point that it may as well not even exist, or you have been unable to satisfactorily expand your business to your standards to meet demand. Either way I sincerely feel sorry for your customers, because unfortunately you can't buy business acumen in the pharmacy.