The humor being used in this scene is rather layered, but is derived primarily from extreme analysis of a benign topic. The various characters are presenting differing personal opinions that they very strongly believe just as strongly as the next character. But there is humor because most people would not give as much thought into the particular point being discussed. Additionally, the point being discussed is humorously unimportant compared to alternate, potentially more important topics they could instead be discussing (such as the actual content of the message).
The General says "Already, you've failed" to emphasize his authority and experience in dealing with women. Josh has just started to text, so the average person would not be assumed to be able to make any sort of reasonable judgment on whether the message would be successful, and the average person would assume the message Josh typed to be acceptable. So by contradicting strongly and so quickly, the General hopes to assert himself as an expert that knows the topic so well that he already knows the result. But there is humor in this because he may not actually have that experience and authority, and merely thinks he does.
So the General's meaning is exact: he thinks Josh has failed immediately. But the reason for his statement is as a disguised boast, trying to convince the others that he is an expert.