I don't agree with this. It's as hard in the real world as it is with text to figure out if somebody is being sarcastic or honest. Usually what we rely on is a bunch of unwritten rules for communication, and those exist for online communication just as they exist for face-to-face.
The only reason why you (and I) didn't read it as a passive-aggressive trolling comment was because we know the person who wrote the comment. Because it said "duffy" and not "slashdot".