Booking.com can hire in-source employees or demand from out-source employees from external companies that can be hired even hourly. Assume you want to hire 50 new members for the customer care department. How do you choose them?
They assumed that the services supplied by out-sources is as good as that of in-sources (FALSE) and that the customers are equally satisfied from both types of employees (FALSE). With these assumptions, they wanted to point at seasonalities in the time series of the received complaints. That is, they suggested that the time series of incoming complaints was trended (because the company is expanding) and presented seasonal behaviour due to holidays, big events and so on.. My "too money-oriented" answer was to manage the increasing trend with in-sources and to manage seasonality (even daily seasonality) with out-sources.