We categorise and name things so we can communicate with each other. So classifications are merely points of common reference that allow us to recognise that we are talking about the same thing. This isn't a flaw, quite the opposite, it is a survival trait, without which we'd have died-out from eating plants that no one had bothered to previously classify as poisonous.

Where it all goes wrong is when we get overly concerned by being pedantically exact about it. Sometimes it's okay to call spiders "insects" when there is no need to be taxonomically precise, just as it is perfectly fine to use the word "bugs" to mean all manner of creepy-crawlies and not just those that are insects of the suborder Heteroptera. 

Saying that Itchycock (or whatever) was the first prog album is merely a point of reference so we all know what we are talking about. When we pedantically pick over it with a pseudo-musicological taxonomy it loses that peg-in-the-ground relationship and when approached like that very few (if any) albums would survive such close scrutiny. So it's semi-arbitrary consensus (by some undeclared common agreement) that this particular album is chosen to be "the first Prog album", and there is nothing wrong with that, any one of a number of albums could have been picked, it doesn't actually change anything. Itchycock suddenly doesn't become any easier or harder to listen to, it doesn't instantly become an album that you are not permitted to like or dislike; and so its status as an album is not affected by whatever label you give it.