This is false. I've worked with many dealerships across different brands and the service department generally makes far more profit than the sales department. This is especially true outside of luxury brands where actual margins on new vehicle sales can be in the single digit percentages, while margins on basic recurring services are huge. See this Edmunds article for a pretty accurate breakdown [1].

[1] https://www.edmunds.com/car-buying/where-does-the-car-dealer...