And your source for this? The datasheet specifies the pin as a high impedance pin, as an input it has 50K of impedance. Even 10ua with a 50K impedance would mean the voltage at the reference pin would be 0.5V less than it should be. Now we don’t know if the “high impedance source” means 50K because the datasheet does not give any specifications for this pin as an output. Hmmm … why might that be?
But there is nothing wrong with connecting a bidirectional pin to another output. It is, in fact, very common.
If the user has a fixed voltage source connected to the AREF pin, the user may not use the other
reference voltage options in the application, as they will be shorted to the external voltage. If no external
voltage is applied to the AREF pin, the user may switch between AVCC and 1.1V as reference selection.
There is nothing wrong with doing it, but it need to be confirmed as ok for the user. so it warns the user, if it was a problem like an output to a power output, that would be an error.
As for this exact chip, it sinks atleast 160uA through that pin when an external reference is used. however it says “shorted” implying the pin can pass more than that,
