I believe he's referring to the hypothesis (I think tested) touted by Bret Weinstein that lab mice breeding has led to strains with extremely long telomeres, a

I believe he's referring to the hypothesis (I think tested) touted by Bret Weinstein that lab mice breeding has led to strains with extremely long telomeres, a feature that makes them resilient to all sorts of cellular damage, but makes them also susceptible to cancer; Bret's point being that they aren't good for either cancer nor non-cancer research. Lab mice breeders may have responded to this idea, but he alleges it's been swept under the carpet, too many studies at stake. Or something like that.