Technically, the allergist is right. And so is your GP.
An apnea event is the closing of the airway in the throat, ceasing all breathing. The closing happens because the muscles relax in our sleep (they are voluntary muscles), additional material (ie fat) helps pull them down. As they relax/begin the collapse, they vibrate in the breeze, causing snoring. As the collapsing continues, the airway is narrowed further and the flow is decreased. This is a hypopnea. Finally the collapse happens (muscle, fat, regular tissues) and the springy airway is closed.
I am going to assume your allergy is nasal. The nasal passages get irritated and swell. Add in snot
phlegm, and the airway can become blocked. Notice it is not collapsed, but blocked. And you can still open your mouth and breathe. Hence it is not an apnea event.
However, those nasal tissues are connected to the same tissues involved in apnea events. Those irritants and stuff are going down the back and into your throat. If an allergy is bad enough, the swelling and irritation can extend to the back of your throat and add to the problem.
Here, I took a photo of something on my allergist's wall a while back:
It shows how interconnected it all is.
Anaphylaxis allergy reaction is when that swelling happens in the throat and cuts off the airway. It is not a wonderful feeling.
So...as long as your stuffiness (swelling, irritation) is limited to your nose, AND you use a full face mask, you should be just fine. During allergy season (right now), I actually go back to my nasal pillows when I am at my stuffiest. I have found that the direct air pressure from the nasal pillows pushes the crap out of the way and keeps my nose clearer. The humidification gets right to where I need it. I blow my nose and use a nasal rinse first thing in the morning and last thing at night.
But if you experience throat issues too, keep an eye on your data. As long as you are sleeping well and the AHI stays below 5, all is good.