Can someone explain to me how this isn’t a huge problem for this study?
It would be nice to redo the study with a large population that is randomly selected. However there is also no reason to believe that the results will be significantly different in direction. Therefore it would also be unwise to discount this result.
If the risk factor is homogenized, then the variable researched can be isolated theoretically.
However, this doesn’t allow you to test for interaction between those variables.
In the study for example, they identify an apparent interaction between ethnicity and the chemicals mentioned; we don’t know what the effect size would be (whether additive, multiplicative, or something else entirely).
(This is without any domain knowledge of the mechanisms of breast cancer to be fair.)