"of the modern world" is ambiguous. You assume that the phrase is primarily referencing modern lifespans; others assume the phrase is primarily referencing environment.

There clearly is some interest in whether or not there are recent environmental factors -- i.e., factors other than lifespan -- which are contributing to cancer rates.

To your more general question, HN is full of programmers, programmers tend to think of themselves as "system" people, and since (in their mind) every discipline of science is just an attempt to understand some system, programmers tend to think that they can comment intelligently on those systems without any prior expertise in them.