This rings a little false to me. Anybody can grab magic items, Monk could grab amulet's, rings, or bracers of defense to break twenty, or any number of other things to increase their combat abilities. But honestly, excluding magic items we get Fighters at 21 (Full Plate, Shield, Fighting Style), Clerics at 22 (Full Plate, Shield, Shield of Faith) Paladins at 23 (Full Plate, Shield, Fighting Style, Shield of Faith spell) and Barbarians at 24 (Level 20, max Con and Dex, Shield)... And most other classes chilling between 13 and 19. 20 AC is good, not absolute top tier I guess, but saying 20 is not good just... doesn't make sense.
It makes sense if you played a monk with an 18 AC at level 1 and still got smacked quite a bit, but didn't have the offense or hit points to take creatures down quick like fighters, barbarians, and paladins or the means to escape combat as easily as a rogue with Cunning Action after delivering a sneak attack.
For Example, to get that Fighting Style means skipping out on Archery, Dueling or Great Weapon Master, sacrificing a lot of potential offense. Casting Shield of Faith costs a spell slot and a concentration slot, and the Barbarian has to use just as many ASI's as the Monk to reach their top tier, and hit level 20.
Now you're talking. All things the monk can't get. An archery fighter gets an 18 AC if he wants it and gets to unload for a lot of damage with a bow from range, while the monk has to do melee to maximize his offense taking hits with a similar AC and fewer hit points for less damage with the limitation of ki. Great Weapon Master fighters in heavy armor, smiting paladins, or bear totem barbarians all do far better than the monk in combat.
Besides, Wisdom and Dexterity are also great stats for the Monk, that is all their damage and saving throws, which is something they will want to increase anyways that will also naturally increase their armor. Monks lose very little pursuing that AC, no more or less than any other class topping their stats does, and they have so many combat tricks already that even if they took zero feats they are still quite versatile.
They do lose out, when fighting stuff that doesn't have anything to do with wisdom or dexterity.
Magic armor and shields don't use an attunement slot. So the armor using classes can get some nice armor and not have to worry about using up an attunement slot. So they can get a ring of protection that stacks with their armor.
The monk stands out only with mobility. They can move better than anyone. Everything else they do can be done better by some other class. Their saves stand out if you make it to 14th level. But prior to that, they're like everyone else.
I played a monk. It was an underwhelming experience in a group of min-maxers. It had its moments like knocking creatures prone and cruising around the battlefield. It also had a lot of drawbacks like getting knocked down quite easily due to a lack of AC and low hit points. And limited offense tied to ki while other classes were generally doing similar damage without any such limitations. Even the paladin novaing was better than the monk going out all out.
It's not a monk can't be fun. I went multiclass rogue and it was a little better once I got Cunning Action and didn't have to waste ki to disengage. In many other areas it was a very lacking class that other classes overshadowed. Often even the cleric with spiritual guardians, spiritual weapon, and a cantrip or weapon was doing more damage. Then again the cleric is surprisingly impressive if you build them right.