He addresses that in the video: other jumpers and the plane are not above him at this point, so odds are the rock came from elsewhere.

The presumes a reliable narrator, but it seems plausible. This is one guy with an in-flight asteroid sighting. Not as if they're crawling out of the woodwork. And the experts called in seem to find this credible. Though people hamming up an act for cameras is also not unheard of.

Given its a wingsuit jump he's likely miles from where they exited the plane. The rock would have a more straight down trajectory as well. You can tell the forward speed when the other jumper comes by.

The plane is well below the skydiver (about to land) when the rock passes by. There's no way it came from the plane.

He says at 1:20 that the plane is below them the entire time. That was my initial thoughts too, though.