I agree with you about this general idea.
This has also been run through the CIA/FBI/whatnot. Any native Arabic speaker (I assume they have one or two on their staff) would be able to pick up grammar mistakes with great ease. I seriously doubt that they would only listen to the tape and ignore the words at the bottom of the screen.
Real or not, I am not sure this tape is significant in any way other than proving the fact that some of the people who hate us are not Arab.
The message is the same, the language is different.
That is my hope too, but mistakes happen, and the CIA is backing off the initial authentication.