This is transitive risk.

Asbestos is clearly linked to cancer > asbestos was found in talc and known to form with talc naturally > risk of using talc that isn’t pure.

The problem is whether talc is pure. At that point it becomes a question of how much you trust J&J to verify that. My guess is that different mined sources varied in asbestos composition and there likely was some contamination that went undetected. But who knows.