The range loop will terminate when the channel is closed. So yes, it'll reach wg.Done (as long as the input channel is closed).

But if you read carefully, "or it receives a value from done, then output calls wg.Done." So will not be closed in case it receives done, needs to wait for c also! In such case selecting on done is useless.

The text just above says

(We'll discuss in a moment how to allow this loop to return early.)

And then a snippet below shows that the final implementation is

 output := func(c <-chan int) {
        defer wg.Done()
        for n := range c {
            select {
            case out <- n:
            case <-done:
                return
            }
        }
    }

The blogpost is incrementally building a working solution.

So comment is misleading then. Missing goto :)