To sound like a joke, as in everyone will find it funny and immediately know it is a joke, and to sound like one specific obscure line that is a joke in some film, and that people don't immediately get. They both sound like a joke, but here "like a joke" means two different things.
Ah. I think you're drawing a reasonable distinction.
Though I certainly wouldn't say everyone (or even most) need to find it funny to "sound like a joke" in the sense you meant. Surely, there are bad jokes that sound like bad jokes. I think it's probably correct to say that most should "immediately know it is a joke".
All of that said, whether that's likely to be the case in this instance depends on a lot of context that wasn't specified.