I'd ask the promoter. My experience with these sorts of events is the terrain used for the MTB portion is non technical, but I assume the promoter is concerned about people just jumping into an off road race without the proper equipment or experience. Can you go ride the course and see if your bike is suitable?

There is nothing that really differentiates a mountain bike, some have 700c size wheels, some have one speed and some have no front suspension. It would be difficult to draw a precise line between a hybrid, cyclocross bike and mountain bike, perhaps tire width would be the best indicator.

In any event, the hybrid will do OK with some fatter tires, you run the risk of flats, mud clogging the frame where the wheels pass through and being undergeared if it is real slow and hilly. If the course is easy, a hybrid will do well, though your center of gravity will be a little high for optimal handling.

Just email the promoter and explain what model hybrid you have and ask if it's OK. I bet you they will be more than happy to take your money, just bring a copy of the email when you register.

I race pretty technical mountain bike races on a cyclocross bike which is really just a road bike with slightly fatter tires, it does very well as long as there are not large sharp rocks or fast rocky downhills that pop the tires.