Oh, we are going to go there are we?
Sure, according to the laws of probability anything is possible. But while there is a minuscule probability of jumping from the top of Burj Khalifa and living I can absolutely guarantee that actually going and doing it means you will die. Because science (aka gravity) says you will
And yet your argument is that it is possible to do exactly that. In a somehow anatomically/spatially selective manner.
No, we are talking about collective genomic inheritance and expression. A hybrid will inherit a full compliment of chromosomes from each parent and so each [somatic] cell will carry both sets of chromosomes and, with some few exceptions, all of the genes from both parents will be expressed in those cells.
You will
never get a hybrid where the "outside" cells are made up exclusively of only ball python chromosome cells and the "inside" cells are made up exclusively of only blood python chromosome cells. It absolutely, fundamentally cannot happen.
Yes well... I can explain genetics in minute detail
Careful there Ant, you do not want to go speaking in absolutes because that is neither scientific or advisable...
Fact of the matter is that you are both right and both wrong. While polyploidy most often does result in infertile offspring there are documented cases of fertile polyploids that have occurred, generally leading to a speciation event. Some quick examples that come to mind are the Grey and Cope's Grey treefrog and the horned frogs Ceratophrys cranwelli and C. ornata along with more blatant examples like the wheat and corn that humans have cultivated for centuries.
The long and the short of it is that, regardless of whatever mystical hand waving you want to try throwing at it, what the vet claimed is simply not possible.