I'm just armchair musing here, and I'm definitely no expert on sound waves, but I wonder if they considered the fact that most airliners have more than one engine. Could the effect also be the superposition of multiple engine sounds?

Those have a fixed spatial distance, too, and the effect would (I suppose) change with the lateral angle to the listener during the fly-by. This theory should be pretty easy to falsify, because then the effect would not occur if the plane's path went exactly overhead.

For that, the pressure waves (sound) coming from the engines would have to be somewhat coherent, or correlated in phase. Since what we're hearing is essentially turbulence, that's not going to be the case.

Planes also have flaps, slats, and landing gear, which can have a huge effect on the sound heard on the ground.