I'm pretty sure it is, because they even mentioned it in the Stanley Kaplan online course I took.
It seems like a ridiculous rule to me too, but they must believe that if you have a jacket with you which can be removed, you could conceal something in there (crib notes?) and look at it during the test--or something like that. Personally, I don't see how anyone could get away with that, because (as they tell you on the way in) the entire testing session is monitored by video cameras and taped.
But get used to the insane, draconian rules. Here's another example: The center where I took the test (in NYC office building) had huge signs in the waiting area saying "no eating"--and there were similar signs in the hallway leading to the test center entrance. Meanwhile, there was no way in the world you could actually go down to the building lobby and outside to eat something, because that would take up MORE than your 10-minute break. So what did people do? Gulp down energy bars in the BATHROOM--the only available place. (At least it was one of those "fancy" bathrooms with a little anteroom in front with just a mirror and sinks. It felt a little less strange to eat in there.)