If they fell asleep they’re in even bigger trouble, because they told investigators they were having a heated discussion, and the voice recorder will tell the truth very quickly. I really doubt they were asleep, because they were certainly wide awake by the time they were explaining things to investigators on the ground, and they would have realized that any story they gave would be compared to the voice recorder.
There should be two hour or longer voice recorders on every flight.
This flight apparently only had the 30 minute one.
The pilots got lucky as the time from waking up to the gate likely took up all of the 30 minutes as the snoring was written over.