The two parachutists did jump from 800ft over the Olympic stadium, but landed on a bridge alongside. As you say, there were quite a few wires strung across the roof, so no options to land in the middle.
What wasn't shown is that a Merlin (also in Olympic colours) flew across the roof a short while later scattering billions of confetti to represent every person on the planet. The confetti raining down was shown, but no indication of where it came from.
