cabin air filter impacts mpg?

Are you sure you don't mean "Engine Air Filter"?
A dirty engine filter will degrade ICE performance. A dirty cabin air filter will just mean less airflow in the cabin!
At the very least, just pull the cabin air filter, it takes all of 20 seconds, tap it out over a trash can, better yet, just buy several, keep them where ever you like, and replace the filter yourself as needed!
The same applies to the ICE filter, about a 2 minute swap out process!

 

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I'd set it aside as a misstatement.

Contrary to popular mythology, a dirty engine air filter should have very little or no impact on MPG of a traditional Otto-cycle gasoline engine. It will cut the available engine power, which is the performance that racers care about. But people who care about MPG are not 'flooring it', thus not fully removing the air flow restriction caused by the throttle plate, so some air flow restriction from a dirty filter doesn't matter. Lack of sufficient power should appear long before any MPG problem.

This is not necessarily true for an Atkinson-cycle engine, which tends to run with a more wide open throttle. But I haven't seen sufficient discussion to understand how much MPG a Prius may actually lose to a dirty filter. While 'everybody knows it', I haven't seen anyone quantify it.

 

I will agree with ICE filter, cabin air, not so much!
At the risk of stretching the imagination, you could say the fact the cabin filter is dirty it will impede the flow of air and thus make the fan motor run harder thus increasing current draw to try to compensate. The amount of power wasted there is so small he wasted more energy mentioning it than your car actually suffers!
At least tap it out if dirty!

 

DUPLICATE ...OOPS!

 

I suppose, if the cabin air filter were dirty enough to restrict airflow, that the heating and cooling efficiency would be compromised, and the engine would have to work harder to compensate. It may not be obvious, but I don't think it's impossible.