That was the prevailing theory but never proved because they never found the Norseman.
I thought some on the plane admitted it.
I saw a snippet on the History Channel, in which a crewman on a Lancaster convincing asserted that he saw a bomb hit a “grasshopper” - small aircraft - when they were dumping their ordnance in a bomb disposal area in the English Channel. Their mission had been scrubbed and they never landed with bombs, they would drop them in designated areas in the English Channel, which were clearly marked on wartime navigation charts.
Miller’s pilot was relatively inexperienced, and the most direct course between the departure airfield and Paris was through the disposal area. The pilot probably calculated that the danger of being hit in the ordnance disposal zone was too remote to bother changing course. He - and many others - may have done it often, and it was probably a common occurrence. He just got “lucky”.
The incident was hushed up to avoid arousing American public opinion against the British, and more justifiably against the U.S. Army, who were actually responsible. Eisenhower didn’t want to have to say “We screwed up and killed Glenn Miller on Christmas Day” by having him fly at night at the height of the Battle of the Bulge to entertain a lot of high ranking officers, while GIs were dying in the Ardenne.
Occam’s razor favors the simple and attested to story. The circumstances support his account, the time of departure and direct course to Paris would put Miller’s plane in the area at the time. The Nazi’s spread rumors about Miller being killed in a brothel and otherwise tried to make propaganda hay out Miller’s death. The truth was actually a lot worse.