I wouldn't call it "so good", I think "good" is more suitable to describe it.
Not only on the table, if you look at in the internet of (xGA) ''expected goal against'' among the PL team we are ranked 3rd place the best. Only below Liverpool & City. Which tells you that majority teams find it hard time to create good chances against us.
https://understat.com/league/EPL
As a unit, we are definitely good defensively and have improved a lot compare to last season. However, over and over again, we keep conceded goals due to silly mistakes that can be & should be avoided. The Spurs game, you can count 5 players that could & should have done better for the goal, 5 of them came from our defense.
Majority people are just jumping the gun when talking about our defense nowdays. I think it's fair to say that we do need a better player than Lindelof to pair with Maguire. Someone who can compliment Maguire's weakness (being slow) and Lindelof doesn't compliment this and dropping Maguire for Lindelof is a stupid thing. However, the fact that defense isn't our priority right now should also tell people that we're doing not as bad as how people overacted badly about it.