This is not true...
They actually have a venom... The reaction people get when bitten is not an alleric reaction to their saliva... The reaction is from being envenomed... No matter how bad a bite can be from a hognose snake, it can never be compared to that of a bite from a copperhead... Even a mild copperhead bite will be many times worse than even the most severe hognose bite...
Besides, in order for you to be allergic to something, your body needs to first be introduce to it...
If you are allergic to bees, it does not mean you will be allergic to snake venom...
There are other factors that determine how bad a hognose bite can be and none of it has to do with allergies...
#1 The snakes size
#2 How long did the snake "chew"... They are a rear fanged species... However, they do not have the most advanced delivery system... Their fangs are not grooved and there is no duct running from the gland to the fang...
#3 Were the rear fangs even used during the bite??? Despite what many believe, a hognose snake and most rear fanged snakes do not have to work to sink their rear fangs into you or its prey... The fangs are set right under the eye, not way back in its mouth... In hognose snakes the fangs are moveable... They are not as well developed like a vipers front folding fangs but they are hinged and hogs can opt not to use them during a bite...
With all this being said, the huge majority of hognose bites do not result in envenoming and you will get nothing but a few holes in your finger as a result of a bite...