Ed, you are perfectly right.
The IR index mark is fo "real" IR, while most of "us" use "deep red" which behaves about like ordinary red light. Even with an 89C filter on MACO IR820 I could see no sign of focus shift using a 150mm APO-Lanthar lens, and very, very little with a 150mm Symmar.
With any film except Kodak HIE and the very darkest filters, ignore the focus mark - except if it is very far from the infinity mark. Lenses are different, some are better corrected than others. APOs should be very close to no correction even in the worst-case scenario.