Clicking the "query planner" link in https://www.mongodb.com/docs/manual/core/aggregation-pipeline-optimization/#std-label-aggregation-pipeline-optimization-indexes-and-filters

leads to https://www.mongodb.com/docs/manual/core/query-plans/#std-label-query-plans-query-optimization where the first sentence is, "For a query, the MongoDB query optimizer chooses..."

It looks like some of the old wording stuck around and needs an update.