You're missing the point. The skydiver is not at rest either. It is not traveling at the absolute speed of 300km/h, it is traveling 300 km/h faster than the diver. Go ahead and count the number of seconds between when he pops his chute, and when the rock flies by, and tell me if that is enough time for the rock to accelerate to a speed of 300km/h relative to the diver while still being above him.
What you're suggesting is that the rock would have had to have been launched directly upward at a relatively high velocity, in order for it to have time to come to a stop, then start accelerating downward such that, when it passed the slower diver, it was moving substantially faster than he was. There is not enough time for that to happen.
It's cool that you're a skydiver, I admire that, but these people have been looking at this for 2 years. The guy in question owns his own wing suit and other gear. The guy in question does not think it came from him. I hope you can respect his experience and acknowledge that your own experience does not necessarily outweigh his. Your'e not the only person with skydiving experience in this situation.
I acknowledge that, yeah, it's possible for a rock to be launched out of a chute if it was packed with the chute. But that's not the situation that I see in the video.