Could there be some other component of fish oil, such as high levels of mercury, that could be the culprit?
Omega-3 supplements, free of all of the other components of fish oil, might be perfectly OK taken at any level.
This article certainly doesn't make anything clear.
This means that the mice already had a disease that made them susceptible to colon cancer. Even if correct, the application of this finding from mice to people cannot be assured, especially those without a chronic inflammatory condition like IBD. As a practical remedy for any concern, I would take Vitamin D3 and mixed isomer E supplements along with fish oil.