In the real world we have things like intonation, facial expressions, body language, and other indicators to denote sarcasm.
On the internet it is very possible and often plausible that someone can very much believe what may appear to a reasonable person to be sarcasm. Having a crutch online does not equate to an equivalent crutch offline.
Anecdotally, neurodivergent folks I know prefer, and some even require, a sarcasm indicator online.