I think it's important to note a confusing factor that can make someone "think" that appaloosas are quarter horses with spots. That factor is that appaloosa is both a breed, with standards and individuality and a descriptor of coloring.
When people say thoroughbred, they know its a breed, never a color description as well, so "Appaloosa" can be confusing. People may post a sale ad for an appaloosa for sale and it's only appaloosa in color pattern, not the genetic blood lines registered for a breed.
Take a paint horse, for example. It's a breed, and a paint can have no color while being registered as full blood paint. Yet my first horse, who was a Piebald, was not a paint as he was not registered and of cross mixed breeding.
So one has to be careful when speaking of a color. Is it a color pattern you speak of, or a color breed you speak of? This article spoke of a color breed that originally had no color pattern, until about 1880. Appaloosa is an old breed first, and horses of differing breeds who displayed the color pattern were lumped into the appaloosa name by the uneducated, (of yesteryear).