Terminal velocity for a small flat rock is not all that fast. 200-400mph: http://www.amsmeteors.org/fireballs/faqf/#8 http://www.amsmeteors.org/fireballs/faqf/#

Terminal velocity for a small flat rock is not all that fast. 200-400mph: http://www.amsmeteors.org/fireballs/faqf/#8 http://www.amsmeteors.org/fireballs/faqf/#8 Considering the skydiver also has a significant speed, that corresponds reasonably well with your estimate. The rock also looks very much like fragment of a meteorite with classic "fusion crust". And probably they would have known if an airliner was in the area, considering they were essentially right above a general aviation airport. And probably airliners would not choose to fly over that area. I think the meteorite identification is much more plausible than anything else.