What is going on with the accumulator in this example from MDN?

Question

So I was going through the MDN docs on the .reduce method in JavaScript and I was having a hard time understanding what is going on with the accumulator (acc). I was expecting the accumulator to populate/group according to age as it looped through objectArray(below). I've attached a picture of the console output and you can see that the acc is already populated/grouped. Any help would be greatly appreciated.

MDN .reduce: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce

let people = [
  { name: 'Alice', age: 21 },
  { name: 'Max', age: 20 },
  { name: 'Jane', age: 20 }
];

function groupBy(objectArray, property) {
  return objectArray.reduce(function (acc, obj) {
    console.log('acc',acc) //line not in MDN docs
    let key = obj[property]
    if (!acc[key]) {
      acc[key] = []
    }
    acc[key].push(obj)
    return acc
  }, {})
}

let groupedPeople = groupBy(people, 'age')
// groupedPeople is:
// {
//   20: [
//     { name: 'Max', age: 20 },
//     { name: 'Jane', age: 20 }
//   ],
//   21: [{ name: 'Alice', age: 21 }]
// }

Console Output: screenshot