I can't speak to Parent's situation, but temp tables are common practice in OLAP workloads across many database engines (personal experience with Postgres, Presto, Spark, Netezza).

At a certain level of query complexity, no query planner is able to accurately predict the characteristics of sub-queries or CTE, resulting in query plans that are ill-suited to the problem. More often than not, materializing CTEs inside a large query into 2-3 temporary tables results in order-magnitude performance as the database now knows exactly how many rows its dealing with, stats on # of nulls, etc.