In the interest of being exact, I said that too much air is entering the throttle body and the usual cause is a vacuum leak. However, the problem could also be that the idle set screw has been set too high. Or the wrong throttle body butterfly has been used (incorrect bleeder holes). Or the throttle body is binding and not closing all the way. Or the throttle return spring is broken or weak. Get the idea?
Also note that it is possible to have a vacuum leak and still pass a smoke test. Examples include a problem in the EVAP purge system.
My point is don't let your thoughts about the dealer blind you to the possibilities. Or assume that everything the dealer tells you is not true.
From your reply above I'm not getting the sense that there's a complete understanding how the IAC works. The IAC can only add air. When the IAC duty cycle is zero, then all adjustment is gone. The IAC simply can not remove air. Hence the reason why the motor must idle slow (or die) when the IAC is disconnected.
IF plugging the IAC in immediately causes the RPM's to go to 2000 and stay there I would start by looking for a ground fault in the IAC signal return line. This would force the IAC duty percentage to be 100% all of the time. This would also cause a circuit malfunction DTC. Again this is only an educated guess. Tests should be done to either rule in or out the theory.
It is possible to do some tests with a common and cheap Volt-Ohm meter (VOM). For example by measuring the voltage across the IAC one can get an idea of the IAC duty cycle. In a working motor, this voltage should change as the PCM attempts to adjust the idle. What the voltage is doing gives clues. Being 12 volts, zero volts, or never changing are clues to what's going on.
Don't discount the value of a good physical inspection using a low tech tool such as the Mark I eyeball. I have fixed many a wring problem by simply looking for evidence of physical damage to the wring harness.
https://en.wikipedia.org/wiki/Visual_inspection
If asked for an educated guess, the real problem is with the IAC circuit itself. When the dealer finished the motor install they couldn't get it to idle. So the idle stop screw was adjusted up just to keep it running.