I didn’t have clear reception, but any ‘light aircraft’ behind Trump is most likely a military helicopter. Look at a map of the National Mall. At the Lincoln Memorial end (W) you run into the Potomac River, with Memorial Bridge spanning it. That’s where the Rolling Thunder riders cross over from the Pentagon that’s just on the other side in VA ... an easy walk.
No private ‘light aircraft’ can get within miles of where Trump is without being escorted away. If not shot down .. of course they wouldn’t shoot ‘em down, because they’d never get that close.
My first thought was about the restricted air space.
Watch the speech replay, the fixed wind light aircraft was clearly pulling a banner behind it, directly over Trumps right shoulder in the background.
Btw, on looking a second time, it appears to be aircraft pulling banners, but the camera angle is looking through Trumps white sheer stage backdrop towards the water. I am not sure what it was. So your probably correct, as I understand the restricted air space is intense in that area.