The amazing thing is that any research has been done on comfey at all. The argument will be made that the sheep in the experiment, which got cancer of the liver by they way, were fed excessive amounts of comfrey, far more than a person would ingest. This is true and is a practical flaw in such research. Also, the other foods would then need to be screened for carcenogen factors so results would be more uncertain. It is true that a normal diet MAY block the effectiveness of the carcenogens.
Nature has many strategies for making way for new generations to carry on evolution.
Generally, things like carcenogens are not linear. You cannot get cancer development at the speed of light if you increase the dose enough. And you cannot be sure zero cancers if you reduce the dose to a tiny amount, assuming a large population. You just can't distinguish the cancers caused by comfrey from other likely causes at small doses. The argument will be made that with a good diet, one is less likely to develop cancer.