I think 'dash s' meant '/s', i.e. sarcastically bold.
Additionally, it destroys the irony for the readers who understand the /s.
(Perhaps '/' is called a 'dash' or something similar in other languages, but it's an 'oblique' or 'slash' usually in English. To lend weight to that theory, I believe 'oblique' comes from it being the oblique/italic form of a dash - so other Romance languages may well give it the same or very similar name to that for '–'.)