They are not saying "our elections are unhackable" they are saying "our already manufactured machines as available to register votes are unhackable". It could be true today. But now that they have thrown open the challenge who knows if future such machines will be or not. IIRC the machines are hardware with no installable software. The issues you raise about chip and firmware vulnerabilities remain. And the Govt of India have painted a target on their backs for the foreseeable future and way beyond.