Thanks for asking!
1. Can you explain what you think went wrong in using the "measuring another predictor" approach in this example? it looks like they did a reasonable analysis approach, so why do you think they got a different answer than what was found in the RCT?
Maybe the subjects took Vitamin E for reasons different from the reasons they took Vitamin C or multivitamins, and the Vitamin E reasons were more related to other unmeasured favorable health habits or other unmeasured favorable predictors of heart disease.
A more jaded explanation is suggested in the footnote. The fact that they used Vitamin C as the falsification test in one study and multivitamins in the other made me wonder whether they did that because it supported their hope or belief that Vitamin E was beneficial.
2, Are you making a qualitative point about this particular method's limitations?
Yes, exactly so.
Tom