When they want to know what an aircraft looks like without the radar absorbing foams and paints they do that during the testing phase, typically on a prototype

When they want to know what an aircraft looks like without the radar absorbing foams and paints they do that during the testing phase, typically on a prototype that simply hasn't had the fancy stuff applied. Anything done to increase radar signature is typically the aircraft equivalent of the reflective belt. I think the author's hypothesis about IR is far more likely and provides much more justification for why and old type (that they've presumably long since had radar information for) would be tested alongside a newer type.