What you say is true, but it is also possible that someone describing the flavor as "sour" isn't experienced enough to have built up the official vocabulary. You see this a lot in tasting cultures of various products, what one person means by X isn't necessarily the same as another person.

It’s more simple than that. Acids are sour. Coffee has acids that are transformed by the roasting process. Light roasts do this less. Some people like that taste and others don’t. Those that do like it often act superior and treat the other group as uncultured.