Their English is actually good enough to properly use sarcasm. “I had the fortune of getting food poisoning last week” would be understood as a joking way of taking about misfortune.
It is. But just because you speak English doesn't mean you use sarcasm constantly. Sarcasm is a lazy way of communicating in almost any language. It reveals lazy thinking and poor vocabulary. I never assume someone is using sarcasm when they could actually be speaking intelligently, albeit with a common grammatical error.