Why is everyone so bothered about this? Everyone had a good laugh and both inflamed rumours about their rivalry even more. This probably did no harm to their former relation to each other.
Rob's double entendre was funny and witty; Lorde's reply is uninteresting. Rob's tweet was hardly trying to insult Lorde's album; he was just making a light joke. Apparently Lorde doesn't have a sense of humor. Shocking!
It sounds like Rob was just making a play on her album title, as in Lana's new single is more addictive than heroin. Probably not in good taste to joke about drug addiction, though. And with the comparisons between the two artists, it could certainly appear to be a slight to Lorde. But I just don't see it that way.