Chapter 1 Optional Lab - $count

When I read the given answer for the optional lab in Chapter 1, I thought for sure this will throw an error. Not being familiar with the syntax, I read
{$count : labors_of_love}
as a count of the labors_of_love field and, thought that, as there was no labors_of_love field, an error would be thrown. I planned to post:
“the way the pipeline has been written, while there are many 'labor_of_love’s, there are no ‘labors_of_love’”.
But it turns out that $count: returns the number of documents in the pipeline labelled with the given string and, interestingly, this acts as a field which can be referenced e.g. if the pipeline is extended with a subsequent stage.