It is nonsense....usually.
Anything sold in California is liable to be plastered with cancer warnings for the sole sake of eliminating liability and insuring conformance with the prop 65 law.
Essentially, if a product contains a chemical known to cause cancer (regardless of the logic, meaning, it may contain a chemical if inhaled causes lung cancer yet is sold in a solid state....) it must have that label to be sold.
It could even mean it contains a chemical known to cause cancer if exposed 24 hours a day for years in a certain quantity....despite the product containing a minute amount...you get it.
Basically, ignore it in this case.