It is indeed arguable what it looks like, but even within chat/IM applications, there is no consensus or convention on should the message be retractable or not. People shouldn't expect one or the other.
I wasn't making an argument either way about retracting messages. I was just surprised that someone would think that the DMs are obviously like e-mail when the UI looks like FB Messenger or Hangouts and message storage and transmission is controlled by a single company. I'm still curious to know how it's like e-mail.
