Essentially it turns out that tons and tons of stuff, both synthetic and natural is mutagenic to some degree, but we tend to think about it as a binary instead of a spectrum. His argument was that some things that are technically mutagenic, like pesticides and fire retardants have a net benefit in spite of this(e.g. pesticides make a better diet more financially attainable which has a bigger impact on public health than a very small increase in the base rate of cancer), but we lump them all together. It's why California's Prop 65 is just completely stupid; we lump fairly benign things like coffee and wine in with things like asbestos.
The quote I always think of is, "if you have thousands of hypothetical risks that you are supposed to pay attention to, that completely drives out the major risks you should be aware of."