A brief skim of the paper also suggests that the data are from different means of measurement. In the case of measurements from the 60s, those came from infrare

A brief skim of the paper also suggests that the data are from different means of measurement. In the case of measurements from the 60s, those came from infrared and visible light, whereas from the 70s on its passive microwave. The paper is proposing a method to use the old data as passive microwave, and thereby extending the sea ice record. So is OP's comparison still valid? (Assuming I'm not completely off base here.)