It's not the definitive measure of whether or not a team is good, but you are wrong if you think it tells nothing.
It tells only so much like most stats do. Stats are important but not the exact science they make themselves out to be in sport.
Should be taken with a massive pinch of salt. In fact I don’t recall a single xG that had parallels with the correct score in a game. I bet more often than not the xG isn’t the same as the end outcome of a match.