He wasn't sincere in the offer to begin with. Outside information gets into Iran a lot easier than a nation like North Korea. This is an effort to feign an attempt at dialogue, so when it's rejected (and it was), opposition within the country can cite that when sanctions re-impose this fall. One thing to watch with the oil sanctions will be how Turkey - a supposed NATO ally - responds. They've already stated they will continue with oil purchases.
@edpc Why would you expect anything different since The Mullahs have managed to get their puppet installed in Turkey just like they did here?