That is ridiculous. A demon could just open a physical gate between the abyssal plane and Astlan and then just walk through it physically. Powerful demons could hide their presence, and wizards are not as competent as they would like to think. They are quite competent in some fields of study, but in fields that involve things such as demons, which is built on false assumptions and beliefs, they find research lacking. When the basis of the system, which is deemed an absolute truth is wrong, it makes it near impossible to truly grasp what is going on. They can get closer, but that fundamental error makes it quite difficult, complex, and lacking.