Based purely on written text? Trust me, inability doesn't have closely as much to do with it as does lack of actual information in communication.
You can't read minds, you don't know what he was actually thinking when he wrote that. You can only assume. Even in a real life conversation, you can never be sure, you can always only assume. Except if a person is a notorious liar and hides their motives with such good acting that you can't tell their true intentions, then you're screwed. But usually it's obvious what people are really trying to say.