You're right, it'll likely be a new Apple Watch.Look, it's more than likely Apple will announce MBP2016 in October. But to say Apple will announce ONLY an iPhone in September is silly. History strongly suggests something other than iPhone will launch at the September event.
iPhone 6S-launched with iPad Pro, Apple TV4
iPhone 6-launched with Apple Watch (initial unveiling) and Apple Pay
iPhone 5S-launched with iPhone 5C
iPhone 5-launched with iPod Nano, iPod touch, EarPods
iPhone 4S-launched with Siri, iCloud, iPod Nano, iPod Touch
iPhone 4-WWDC stuff
iPhone 3GS-WWDC stuff
iPhone 3G-WWDC stuff
iPhone-launched with iTV