I think the point here is not which prior to use for effect of chocolate, but how to account for the differences between actual and self-reported chocolate cons

I think the point here is not which prior to use for effect of chocolate, but how to account for the differences between actual and self-reported chocolate consumption, which itself might be correlated with cognitive ability. Maybe people who ate chocolate but did not report it (because "eating too much chocolate is bad for you") tended to have lower cognitive ability - they did not realise that by reporting incorrect intake they introduced bias in the study.