When the leaks of the iphone 7 showed up last year, it turned out to be accurately close to what they revealed on Keynote. They have about 3-4 months to produce the devices otherwise, the apparent rumors of a delay leads me to believe they're trying to find ways to buy more time to implement the Touch ID under glass. And if that was the case, something tells me somebody has been slacking over there behind schedule.
If they don't unveil the new phone by September on time, they have a lot of explaining to do because they have to get moving to bring the new iMacs out by the end of the year like they said they would via their ' apology PR interview ' ( remember that one? ). They can't cram the iPhone and iMacs this close by the end of the year.
But I think you're right because they have to start manufacturing the devices soon before Fall or end of summer comes around. After all, they have to create some marketing and advertising in advance to support it. All this cannot be done at the last minute.