There is a minimum flying altitude for aircraft over the city. One presumes the helicopter was above the minimum. The bottle rockets I have would never reach that minimum so whether or not they would rise through the down wash is moot.
As for the down wash itself, it's a WAY too common misperception that wings work on the Bernoulli principle, one even sees this error in high school text books. The Bernoulli principle has almost nothing to do with it, the idea that air flowing over the curved top of the wing prodouces a low pressure area has almost nothing to do with it. If that was the way wings worked there would be no ground effect, airplanes wouldn't be able to fly upside down, aireobatic planes wouldn't have symmetrical wings, helicopters would be able to hover in free air at high altitudes, and paper airplanes wouldn't be able to fly at all.
A wing prodouces lift by accelerating air in the opposite direction. Stand under a helicopter and you'll notice the downwash, or behind a propeller and you'll notice the back wash. You also have to account for a marine propeller operating in water, an incompressable fluid.
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