Absolutely.
If al Qaeda really had significant help from inside the Saudi security apparatus it is highly unlikley that al Mukrin's body would now be lying rotting in the sun denied prompt burial. He'd be long gone and safe.
If al Qaeda really had significant help from inside the Saudi security apparatus it would be the very last thing they would publicize. They aren't stupid people: misguided in the extreme, but not stupid.
This statement is clearly intended to sow distrust within the Saudi security apparatus and make it less effective right now when they are making real inroads against al Qaida.
It is highly suspect for these reasons.
It could also be highly credible for the reasons you stated. Just change the intimidation to the royal family. This might be the opening salvo in a palace coup.