They use Markdown-esque syntax for message formatting. It mostly is Markdown, with a single caveat that gets me a lot: *Single asterisks* become bold, instead of italic. For italics, you have to use _underscores_.
Other syntax works fine. Slack also uses Markdown. In summary that's nice as it is just one single syntax to remember across systems. I wouldn't enjoy too much that every app started having its own different, more expressive syntax, at least not for apps which their primary job is communication, not text edition.