I vote bird. The slow motion video freeze frames show the wings in different positions. It's also too small to be an aircraft. Compare to the size of the Harrier. Also, it looks like the drone is actually moving more slowly, and they sped up the video. This makes the bird look faster. You can see the cloud shadow move too fast in the raw video. And the butterfly. This explains why there was no sound, or shadow, or wind effect on the trees.
No steel in them at all. Structures are composites with carbon-carbon or UHTC leading edges and flight controls. Times of flight are quite long after reentry.
