Had same issue and got similar reply from them. I replied back by sending screenshots of the promo page from jet website and also the promo email which i received. I didn't get any reply back but in a week or so when I checked my account there were no miles expiring. Subsequently I also called the jetprivilege helpline and they confirmed validity has been extended by 2 years.
Apart from replying to their email you should also call the helpline directly and explain the issue. They also take the complaint and register the issue which can help.

Vishal