Yeah it's likely a production delay as to why it hasn't been out yet. Another reason could be that he drove one when there weren't cameras around.I am not sure if Jay Leno got a formal drive in the Hummer yet, or just got shown the show truck? Jay is getting a bit old and forgetful. We will have to wait and see, but I would guess Hummer test drives have not started yet, more likely test drives will be in the last week of August and into September. Any embargo would lift in mid September before the Texas State Fair.
We know the first street legal HummerEV was built 2 week ago, which would be the first one Jay could legally drive. I do not think even Jay gets to drive prototypes, because if something goes wrong they cannot be insured.
It usually takes Jay Leno's garage about a month to produce a show after filming, so he likely drove the CT5V Blackwing a while ago.