We're talking about social anxiety, not social skills; they are anticorrelated but one is not the same thing as the lack of the other. Someone might have no particular reluctance in interacting with others but still be unfun to be around (low social anxiety, low social skills), and someone else might instinctively flinch from social interactions but if they somehow overcome that (e.g. by drinking alcohol, or by the interaction being initiated by the other person, or by being already friends with them) their company is very enjoyable (high social anxiety, high social skills).