Whether he did pull you over or not, it wouldn't matter. He couldn't even answer the question because he doesn't have the details of your stop. They're rare offenses yes, but certainly not one in a million.
As of a defense of having a radar in the car, take a look at the section:
79 (2): No person shall drive on a highway a motor vehicle that is equipped with or that carries or contains a speed measuring warning device. 1996, c. 33, s. 12.
So basically, if you have a radar detector in the vehicle, you will get charged. The exception is:
Subsection (2) does not apply to a person who is transporting speed measuring warning devices in sealed packages in a motor vehicle from a manufacturer to a consignee.
So unless you were driving a delivery truck with it sealed in it's packaging, it does not apply to you.
If you were thinking of fighting the charge based on the officer not being allowed to search your car,:
A police officer may at any time, without a warrant, stop, enter and search a motor vehicle that he or she has reasonable grounds to believe is equipped with or carries or contains a speed measuring warning device contrary to subsection (2) and may seize and take away any speed measuring warning device found in or upon the motor vehicle.
If you take it to court, it doesn't matter if it is valid in your own state. You want to drive on our roads, you play by our rules. They have the evidence they need against you (confiscated RD Unit) therefore it'll be a slam dunk for the prosecution if they take it to trial. You could probably get a reduced fine, but at the end of the day, is it worth it to take a day to drive to Canada to get a reduced fine. IMHO I would just pay this and forget about it. It's only a monetary fine, and has no insurance impact since you stated before, Ontario doesn't have an agreement with Arkansas.