I would suspect the multi-function stalk also but I can't find and don't seem to remember anyone ever having a horn problem associated with a light problem. I looked thru the wiring diagram and don't see how they are related. The only thing I can think of is when the multi-function switch goes bad,it actually melts parts together inside the stalk because of the heat generated when the lights are on (shown to some of us by an expert mechanic (MX-5) on another forum). Usually, the head lights/fog lights stay on all the time because of the melting. The horn wires go down the steering column along with the wires related to the multi-function switch, maybe even in the same wiring harness. Maybe somehow the malfunction of the switch is somehow causing a short in the harness that makes the horn blow.
Only guessing. The dealer will have to figure that one out, but they'll probably just replace the stalk and everything will be back to normal.
