Or it could be that the people that are not necessarily healthier, but have a history of cancer in their family, may be more likely to respond to a colonoscopy invitation? One way or the other, this is all just guesswork...
I feel this is a really reasonable take. This study doesn’t say anything bad about colonoscopy, it says that it is possible the effects are not only due to the colonoscopy so it is probably between 0 and claimed effect effective. Can’t really say where in the range from the study.
The study answers a question from health policy makers: Should we invite everyone (in some age group) for a colonoscopy? Based on this study, probably not.
It does not answer questions from individuals: Should I get a colonoscopy? If you have some good reason (symptoms, doctor advice, family history), probably yes (based on other studies, not this one).
To the extent that there’s a spread in those answers, a third possibility emerges: how can we, as health policy makers, find a way to make the invitations more effective at converting to administered colonoscopies?
Why would that be a goal? First you'd need to show that colonoscopies were helpful in general, or that invitations were going to a subpopulation in which colonoscopies were helpful.
If colonoscopies are helpful to avoid negative outcomes, but invitations to colonoscopies are not, looking into making invitations better seems like an obvious play to me.
The problem is you can't run a study on colonoscopies that doesn't involve an invitation without forcing people to get a colonoscopy at gun point. Which would both be illegal and never pass an IRB.