Part of the big issue here is that they're:
A. Dragging up something that was filmed before February of last year and wasn't even used, just to sell some papers.
B. Relying on people's hearing. I'm no psychologist, but its been documented that if someone tells you to listen out for something, chances are your mind subconsciously hears it (even if it is nowhere near or at least very vague) because thats what you want to hear. Tie that in with people who don't like Clarkson, and of course people are gonna say he said it.
Of course, that argument works in reverse; TG fans who don't want to believe it will just shrug and say it doesn't sound like it at all. As for me? It just sounds like random mumbling in my opinion. Sure, it could have been him slurring the speech to sound indistinguishable, but given what Clarkson said in his statement video, that was the idea - to just make it unintelligble and thus, cause no offence. And to be fair, unlike most of the half hearted apologies the TG crew have to give to people, this one does seem like he takes it seriously and wants it addressed.