Using a reference with threatening language in it that is potentially unknown by the people recieving it, makes a death threat even more sinister as it feels like you're building in plausible deniability and trying to have your cake and eat it.
It's basically admitting "if I just said this it would cross a line, but if I quote it instead then it'll not cross a line for people who know it's a quote, but still cross the line for the people I'm threatening. So I get to make the threat and disclaim any intent at the same time.
The first is that Tan seems to have thought that people who were upset by the statement were unaware that it was a song lyric.
The second is that some people think that because it's a song lyric, it somehow doesn't count.
I can call your mother a hamster and that your father smelt of elderberries; just because it's a quote from Monty Python doesn't mean it wouldn't be insulting to you.