I tried to write optional Lab quiz for $match and $project stages, but noticed that advanced expressions are required that were not touched upon. I also accomplished M001 basics course previously, but none of $setIntersection or $gt […] were explained at all, so I can’t figure out how a beginner participant can afford it.
That’s why the lab is optional. It requires the participant to dig in the documentation by him/her self.
As @ steevej-1495 says, the reason this is “Optional” is that some of the required techniques have not been covered yet. As far as ‘$setIntersection’ goes, the documentation is here. However, you should I think be familiar with all the standard query operators such as $gt, and the aggregation query operators are pretty much the same. Look in the docs for all these.