This is not completely true. A person's reaction to the mild toxicity of hognose venom is dependent on the person's anaphylactic sensitivity to the venom... in other words, allergy.
This is just like a bee sting. People have varying reactions to bee stings depending on their anaphylactic sensitivity to Hymenoptera stings. Those who don't have this type of allergy usually experience just the normal ouch and some itching or mild swelling probably if bit multiple times. Some people with the allergy stops breathing from shock.
But you are correct in that just because you are allergic to peanut butter doesn't mean you are allergic to bee stings, and that just because you are allergic to bee stings doesn't mean you are allergic to hognose venom. Allergies are specific to "type". So that if you are allergic to bee stings you are also allergic to wasp and fire ant stings because they are all Hymenoptera stings.
My sister, for example, is extremely allergic to shellfish, penicillin, and Hymenoptera bites. But she is not allergic to Fel d 1 or any of the cat allegens nor peanut butter nor milk. My son is mildly allergic to dust, peanut butter, milk and Hymenoptera stings but he has no allergies to shellfish nor penicillin... But he is extremely allergic to Fel d 1 in cats. I am allergic to dust and pollen and nothing else.
A lot of times though, a person who has allergies usually have allergies in multiple types, so that when you have an allergic reaction to something, it would benefit you to get tested for any other type of allergy. But, like you said, snake venom allergy is rare therefore, I don't know of a place that tests for that allergy - I've tried looking.
Hope this helps.
My understanding is that there's not a single instance of anybody going into anaphylactic shock from hognose venom - but that can be because hognose bites are extremely rare, not necessarily that nobody is that extremely allergic to it.
Here's more info on hognose venom:
http://www.hognose.com/pages/venomous.htm