"That phrase is just a shorter way of saying "opt-in plus confirm". If a website gets a request for adding an e-mail address to their list, sends a "confirm that you really wanted this" e-mail to the address, and doesn't send any more e-mail unless you click the link and confirm, they definitely aren't a spammer."
That is opt in. There is no plus, this is the minimum required for an opt in list.
If you just put up a form that says 'add me' and add them that is NOT an effective opt-in, that is simply blind spamming. This is because anyone that knows (or can guess) your email can sign you up for anything without you actually opting for this in any sense of the word. So the 'confirm' is not some sort of extra requirement, above and beyond opt-in, it's *an integral part of the opt-in process*.
"Honestly, anybody who has a true opt-out that really stops e-mail isn't a spammer"
Wrong. Anyone that sends spam is a spammer. Spam is unsolicited bulk email. If you are sending bulk email to people that you do not know for a fact actually signed up to receive it then you are a spammer. An opt-out link after the fact, even if it would hypothetically work should anyone be stupid enough to click it does nothing whatsoever to change that fact.
You know you should never click those, right? that just confirms the address is read. EVEN IF they take you off that one list, they turn around and sell it to the other spammers as a premium address at that point and you get on a dozen other lists instead.
Really, use your brain and think about the consequences if what you said was true. I would be able to sign you up for mailing lists all day, every day, and you couldnt do anything to stop it other than change your address. And as soon as I found your new address it would be in the same shape.
Even if every remove link worked, and even if using it didnt just get you more spam, it would STILL be unreasonable to expect you to spend all day unsubscribing to all the crap I spend all day signing you up for. And it's still absolute nonsense to claim YOU opted in to anything when I put your email in and you were never asked whether or not you actually wanted it in.