I believe the ES6 part of the curriculum got moved up in the order at some point. So parts of it may well refer to some knowledge you would have gained before it, in the old order.
It made sense at the time ES6 was added to the curriculum to have it after the ES5 code and then show how you would do some of the same things in later versions of ES. But then it got to the point that it was weird how late in the curriculum current syntax got introduced. So it got moved up (afaik).
There are constant updates to the language and the curriculum. A new version of the curriculum is also in the works.
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