Bottom line, this type of study does not prove cause and effect. If I did a study on sunrise and humans waking up, I would find 100 percent correlation, but that doesn't mean that the sun came up because you woke up. Correlation, yes; causation, no.
Is it plausible that higher omega-3 intake increases prostate cancer risk, but decreases breast cancer risk? I suppose the subtleties of carcinogenesis might allow for it, but I find it very far-fetched; if it doesn't stretch the envelope of credibility to the tearing point, it sure comes close.
The first thing you need to know is that no fish oil supplements -- or any other kind of supplements, for that matter -- were given in this study. None. This study looked at blood levels of long-chain fatty acids such as those found in fish (EPA and DHA).