Eh? Jupiter is 'truly unique' because it doesn't have an extremely puzzling feature that only one other planet is known to have?
Personally, I don't understand the people defending the usage here. As far as I have experienced in my life, when people say "unique" in good faith they mean "one of a kind" and those people who don't use it that way are considered to have done a bait-and-switch.
That said, perhaps there's been a lingustic shift I haven't yet noticed.
No, the article is actually saying that the lack of a hexagon is important. Combined with such insights as "some clouds are higher than other things", the mentioned items really don't add up to a breathless "truly unique". The things we already knew about Jupiter - the vibrant bands and the red dot as obvious starters - are far more interesting and worthy of a discussion about what makes the planet unique.