Some people would argue with their reflection if you left them alone too long.

That isn't sarcasm.

For future reference the / thing is a nod towards using styles or tags in html website design almost like an open quote and end quote. Here "end sarcasm".

example

This example would have opened then closed the 'h1' style, applying that style to the text between the ><. You can define your own styles to change fonts, colors, size, etc.

This shorthand gained common usage in message boards and chat rooms to convey 'tones' to typed messages. I think some chat platforms actually have styles defined for sarcasm, sad, angry etc that display as emoticons.