Pereira, Chilwell are key players too. The EPL's leading scorer in Vardy, and two CBs that has heavily contributed to the best defence in the league. I don't see how Maddison is any more important than the fullbacks, the CBs, Tielemans and Vardy.
The Leicester defense gets protected by the most proactive DM in the league in front of them. They have less to deal with than if they didnt have Ndidi at DM. For example last season for most of the season Ndidi didnt play DM, he played further forward and Mendy played at the deepest point. Their defensive record was nowhere near as good as this season. Thats not to say that Soyuncu isnt playing well himself at CB, but he'd have a lot more to do with any other DM in the league in front of him. Thats why Ndidi is a key player and has the biggest impact on the defensive record. The most tackles of all DM/CMs in the league and the 2nd most interceptions. So thats how they get the ball and alleviate responsibility from the CBs.
What do Leicester do with the ball?
Maddison averages 56.3 passes on average per game when he plays CM. Thats 2 less than David Silva, but 3 more than De Bruyne. So thats a very good amount for an attack minded CM, he's keeping up in that respect with the two players most people would say are the best AMs in the league.
Also he averages 2.8 key passes from those 56 passes, with Silva 1.7 key passes and De Bruyne 3.5. Not only does Maddison play the most passes of Leicester's midfield, but he also creates the most and is their 2nd top goalscorer with 3 goals in 6 games in CM. 3 goals is joint most in the league for a CM despite only playing 6 games there so far this season. Silva is one of those joint with him on 3 goals, just with more games played.
Maddison also has the 2nd highest amount of dribbles of all English players this season behind Sancho. He is still behind both Silva and De Bruyne in successful dribbles, but he's still somewhere up there and its clearly a part of his game to take players on.
Maddison also has the most shots out of the Leicester team with over 3 per game.
So thats all the great stuff he's doing when Leicester have the ball, he's clearly pivitol to Leicester keeping the ball and creating chances, but also getting into positions to shoot himself. Thats why he's the key attacking player, however lets go back to when Leicester dont have the ball.
Maddison also averages 2.5 tackles per game in CM, as well as 0.3 interceptions. So he's also the best ball winner of Leicester's midfield other than the main man, Ndidi. So he's helping him with that pro-active defending the most.
So thats why those are the 2 key players. Ndidi is huge for reducing the pressure on the CBs, Maddison helps a bit with that but mainly he gets the ball moving the most with passes for Leicester as well as creating the most chances, making the most dribbles and having the most shots.