Only guess I can make is based on experience with 2 series cars. In LH2.0 and LH2.2, the IAC is a three-terminal radial floating solenoid valve driven by a transistor on each side supplying opposite phase variable duty cycle to work against the inertia of the armature to set the air flow. When someone accidentally connects the identical three-terminal connector to the throttle position switch, it connects the 12V center terminal through the switch to one of the two driver transistors, permanently damaging it.

It hasn't happened to me, but I've read many unfortunate stories, and helped some through replacing the transistor. I don't know whether you can diagnose it by the lack of throbbing, but the symptom has always been super-high idle speed, making it clear the TPS always whacks the transistor that would pull the valve more closed. A scope or LED test probe could prove if you don't have another ECU to swap in. If you believe the connector accident could not have happened, I don't know what to suggest for you.
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Art Benstein near Baltimore

After marriage, husband and wife become two sides of a coin; they just can't face each other, but still they stay together.-Sacha Guitry