An x ray would go the extra mile in showing if it is actually genetic though. I am sure he is knowledgeable, but without x rays or cultures, you can't guarantee it is malocclusion, as abscesses, cracked teeth, mouth infections and even food stuck in the mouth can all cause malocclusion like symptoms.
Did he culture the saliva?
Sometimes chins also get infections in their mouth which can cause the teeth to grow in oddly and cause what appears to be malo with the drooling, tooth troubles, and can even cause their mouths to smell stinky. It mimics malo and without a proper culture with their saliva, one would think it is malocclusion.
With antibiotics however, you can sometimes get rid of the mouth infection and cure the problem if you are quick to treat it. I had one chinchilla many years ago with a mouth infection from his water bottle tip (the only thing my vet and I could find conclusively that could have caused it. Since it was sharp where my chin had bit into the metal tip.). Everything looked just like malocclusion, but the culture of his saliva proved it to be an infection, that eventually set into the bone, and we had to put him down after 6 months of fighting his on and off again infection.