Electronic throttle bodies have an idle learned value based on the operating conditions of the engine (such as how much air is entering the intake).
For example, if the throttle plate gets covered in carbon, the flow rate of air decreases, and the car starts to run overly rich. The ECU increases the throttle position until it hits the desired Air/Fuel ratio. It then commits this new value to its memory.
Your skepticism is not misplaced, as dirty throttle bodies usually have the opposite effect, causing too low of an idle and stalling, but it is possible that buildup is preventing the throttle plate from closing effectively. In my mind, cleaning a throttle body is free, and doesn't take much time. I also think it should be part of a maintenance routine, as it can affect performance.
The other thing I am thinking is it may be that the throttle learned values got reset. Did you recently have a dead battery, or disconnect the battery for a while?
That would cause all of the throttle position values to reset and possible give you weird idle speeds.