This is clearly false, as both irony and sarcasm are usually delimited by both changes in speech and nonverbal cues. The better you know each other, the fewer of such cues are necessary, but especially among 'new' people, you'd better make sure they get you are being sarcastic/ironic, lest they get a completely wrong impression of you. The lack of cues and subsequent misunderstandings is exactly the problem with speech on the internet and is exactly why things like italics exist: to be able to shift emphasis like you'd do with speech or nonverbal cues.