Severely low xG conceded doesn’t mean “defend” more of the game, on the contrary it means you have a better control of the game. You are essentially allowing fewer opportunities for the opposition to have a glimpse of your side of the pitch. Teams that defend more, often concede a big xG, and if they have good defenders then their xG per shot would be lower. But yeah, pure low xG is all about controlling the game and the ball.