This is true. I actually think he catches the top edge of the thumb hole with the sander on purpose.
Then what he is doing is NOT bevelling your thumb hole. Just because he's using the bevel sander doesn't mean that what he's doing is bevelling your thumb hole. Bevelling is rounding off the sharp edge of the thumb hole.
When you're asking your question "how much bevel do I need?" or "how much bevel do you have?" I think that you're really wanting to know how much adjustment should be (or has been) made to my (your) thumb hole with the bevel sander.
These adjustments are very personal and change with each bowler. That question can only be answered by yourself, working closely with your ball driller.
As regards to span, the method for fingertip drill to say that the first crease should be in the middle of the hole was taught back in the day (when everyone had a stretched span because they needed to 'hit up' on the ball). Because people no longer need to 'hit up' on the ball, the stretched span was found to have more inconveniences and was left by the way side in favour of a relaxed span where the edge of the grip should be midway between the first two creases.
When I first started bowling (ah, the good ole days....) finger grips didn't exist. The finger holes where only drilled as deep as you needed them. Because of this it was impossible to put your fingers too deep into the finger holes. This is why, now, you should always ensure that your thumb goes completely into the thumb hole because if it is not correctly inserted, then you may have your fingers inserted too far into the finger holes, causing inumberable problems.