[ptero]

First, even if the transponder weighs 100g mounting it securely on the drone will likely add more weight and affect flight. Ditto for reliable receive / reply.

Even if it were equipped with the transponder it is not a given that F-15, as flown, would be (A) interrogating it, (B) show the pilots the replies, (C) show the pilot the replies early enough to make the difference. Pilot flying F-15 at 500AGL is likely training for other things than avoiding small drones. I think at those altitudes and speeds he is given the clearance to exercise (500+kts; not the regular restriction of under 250kts below 10000ft) in a well defined region and assumes that it is clear.