Though I do not know the situation AT the firm you were interviewing in, if there is some unexpected increase in data volume OR say a job fails on certain days or you need to do some sort of historical data load (>= 6 months of 1 gig of data per day), the solution for running it on a single VM might not scale. BUT again, interviews are partially about problem solving, partially about checking compliance at least for IC roles (IN my anecdotal experience).

That being said yeah I too have done some similar stuff where some data engineering jobs could be run on a single VM but some jobs really did need spark, so the team decision was to fit the smaller square peg into a larger square peg and call it a da.In fact, I had spent time refactoring one particular pivotal job to run as an API deployed on our "macrolith" and integrated with our Airflow but it was rejected, so I stopped caring about engineering hygiene.