I don't believe it, NotPetya was generic ransomware that spread to a lot of organizations including the NHS in England. This fiction, yet another example of the neocons attempting to demonize the Russian Federation, no doubt to distract from problems at home.
Whether insurers like AIG can run away from their contractual obligations playing the "cyber war" card is a different issue. Technically, it was a cyberattack similar to many others, no matter if the authors were Kremlin-employed or not.