His example is not correct, and there are other examples on his page that are wrong

You can add a third item, set the background to transparent to hide it
and then set the box justify-content: space-between

body{
    background: rgb(44, 44, 44)
}    
.box {
    display: flex;
    width: 200px;
    height: 200px;
    background: rgb(225,225,225);
    border-radius:10px;      
    border:outset white 10px;
}
.item{        
    width: 30%;
    height: 30%;          
    background: rgb(44, 44, 44);
    border-radius: 50%;
}
        

.item:nth-child(1) { 
    margin: 5px 
 } 
.item:nth-child(2) {
    align-self: center;
}

The style is not added, buddy