You have to choose between the ATTiny85 part of the circuit and the reverse polarity protection part -- you can't install both parts because they will interfere with each other. If you install the ATTiny85 part, you won't have reverse polarity protection.
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So you are correct that if you don't want to use the ATTiny85, you must install the reverse polarity components (Q1,D5,R16).
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Cheers,
Oliver KI3P
