Well, Tyrol has been suspicious that he was really a Cylon since the second season, including having recurring dreams about it. The only reason he stopped suspecting it is he was talked out of it by Cavil, with the line that is amusing in retrospect about "I'm a Cylon and I've never seen you at the meetings". Also, Tyrol being one of the Final Five makes sense since he had an uncanny sense for knowing where the Temple of Five was on the Algae Planet.
RDM said that the "final five" are fundamentally different than the other Cylons, including that they are unique and don't have copies. I think the "final five" aren't bioengineered and cloned machines ready to download, they are "Cylons" in that they are children of the Cylon god like the Cylons, but not physically "Cylon" and their bodies are human, they came first (since we know Tigh dates back to the original Cylon war, he was a kid when the first robot Cylons were rolling off the assembly line), and have a completely different goal (protect humans from the rest of the Cylons? Ensure that the prophecy proceeds as it must by balancing the Humans and the Cylons?) The song they were hearing was like a signal from their god, just like Roslin's visions are a sign from hers. They may well have been born to human parents and be biologically human, but divine intervention makes them "Cylons" in that they are affiliated with the Cylon god instead of the normal pantheon of the Lords of Kobol
I've long said that I think the Cylon God is a fallen or renegade member of the Lords of Kobol, as was implied in a cut scene from Home, Part II. The Temple of Five, built millennia ago by worshippers of the Lords of Kobol but apparently being sacred to the Cylons and dealing with the Final Five, just plays into that.