No, I don't buy that - the fingerprint sensor (and it's possible location) has been talked about so much, if a potential "leaker" had the actual home button in front of them and it was just a button, we would see it - that news would get out.
I think it's pretty likely that it's happening. The 5S needs something to make it special. Even the 4S (much maligned when it was announced) had something unique in the form of Siri. And people often forget that Siri also required new hardware - a custom noise cancelling chip and an all new and additional proximity sensor - that was new and exclusive to the 4S when it was released. As far as I remember, while the theory of integrated Siri was rumoured before the 4S announcement (an educated guess since apple had bought Siri's developers) none of that hardware was leaked or shown ahead of time.