Closer, but not entirely true-
Mileage of individual cars is directly related to both span of service, and consistency of service. Having a high peak frequency but low off peak frequency results in lots of trains sitting around for much of the day getting little use. Having a line with consistent service, regardless of whether it is short and infrequent, or long and frequent, results in trains with high mileage. Short vs long, and frequent vs infrequent both affect the number of trains you need, but not so much the overall usage of those trains.