Time series problems are indeed temporal, but not all temporal problems are time series problems. Time series deals with time-specific features of a discrete sequence like autocorrelation, trends, seasonality, etc. whereas frequency domain methods deal with, well, frequency.

Support you’re are looking at sales patterns over a long period of time, which has certain patterns. FFTs are unlikely to tell you much that is useful or predict much whereas time series methods can reveal patterns where the t is the independent variable.