I think it might be to answer the "Apple isn't innovating" rumors.
The first iPhone was announced 6 months before it was available for purchase. They could absolutely pull the "one more thing" deal with the iWatch and have people talking for half the year and NOT buying products from anyone else while they get revved up for release.
Who knows?
I kind of agree, in that whatever happens, I just can't see Apple missing the pre Xmas selling window.
The iBand/iWatch will (I think) be an enormous seller for Apple, but will it sell as many if it misses that window?
I don't think so, as I think this will account for a huge amount of impulse buys, as it's not something that needs anything complicated like a phone contract and it's not a phone that needs to be purchased on the right network, etc, you just go in/online, buy the unit and you're done.
I do think you're right about an 'unveil' in September though, to get it out there and ensure anyone who's on the fence about a wearable will hold on 'because Apple will be releasing one' real soon.