Can someone help me understand output impedance to an ADC?
Andy's answer gives good information about how to design your circuit.

Let me try to address what you're misunderstanding about the ATtiny spec.


  The ADC is optimized for analog signals with an output impedance of approximately 10 kOhm or less.


This doesn't say anything about the output impedance of the ADC.

It says that whatever circuit you use to provide an input to the ADC must have an output impedance below 10 kOhms.

Andy's answer gives a good overview of how that applies to your situation.