The two species do indeed overlap in many areas, so they "do" have the possibility to cross. Similar types have done so in captivity, so the physical possibility is there.
Now consider, humans see only a very very small percentage of any population of any species, including such things are waters and garders. Muchless a subterrain(fossorial) species like zonatas.
Then consider, 9 our of 10(or thereabouts) of all zonata hatchlings perish/consumed/die before adulthood. Then consider, a cross can be even less suitable for survival. Lets say, 1 in 50. The chances of humans seeing one of these would be astronomically small.
The point is, its possible. Not common, not probable, just possible.
Your broad statement of denial. Paints a complete picture of it never occurring. You understand, never, has a far greater chance of being proven wrong, then possible does. Remember, never is a long time and includes many conditions we do not understand. In fact, it includes all conditions, past and future.
Possible includes things we do understand, climate changes, behavioral changes, habitat changes, etc. And things we do not yet understand. No one here is saying that its common.
To add to this fire, two(that I know of) wild caught gophersnake/kingsnake crosses have been brought to the San Diego zoo. Now consider, gophersnakes and kingsnakes live in and amoungst people, lots of people. So the chances of seeing this is much higher, yet only two. Zonatas habitat, is only recently being encroached upon by people. Most of their perfered habitat is not very suitable for housing tracts.
I did indeed view one of these crosses and as expected, was a perfect blend. So yes, its possible, and that was the question.
Now I have to ask, is everything you have not seen, not possible? FR