I would be very surprised if they closed up at 7 p.m. in September, for a number of reasons.
First, which the OP pointed out, is that we're only a few weeks away, and most people already have their plans based on the posted hours. It will also seriously cause issues with dining reservations made in the park after the 7 p.m. hour.
That is why I put it squarely in the rumor category. I trust my source, but he isn't infallible. But Disney has been toying with schedules all year and they have made cuts at MK this summer ... albeit generally taking a midnight down to an 11 p.m., which doen't affect dining.
But anything is possible ... just look at the thread regarding DAK's hours issues this month.