Jeez, no-one's saying it was made up. Just curious as to how the argument was heard in the cabin, that's all.
An aviation professional on board of the aircraft reported he heard the shouting throughout takeoff and departure for about 15 minutes
Another passenger reported he heard shouting from the flight deck during takeoff and believed the aircraft had been hijacked.
This is just a wag, but maybe the argument was heard because the pilots were....shouting? Like the kind of shouting that was shouted so loud you hear it through noises that your brain automatically gauges are too loud to be pierced by conversation-level (or even mildly-raised) voices.
This is not unlike seeing a guy across a river on the opposite bank where rapids lie between you and he and create a general atomsphere of noise. He tries to tell you something by talking but all you see is his lips moving. But then, he's bellowing and although you can only hear his voice faintly, your brain computes and says "Wow, that guy must be shouting SO loud if I'm hearing him through all this racket".
Later in the telling, you wouldn't describe the encounter as some guy "faintly speaking" to you, but rather as "A treaked-up meth-head shouting at me to get away from his lab. Good thing there was a river between us or I'd have had to shoot him".
That's how I see it anyway