Message system or messaging system?

In a reading passage in our ESL textbook it says:

Kublai Khan made use of a fast and simple message system. Horse riders spaced every 40 kilometers allowed messages to cover 500 kilometers a day.

The exercise that follows on taking notes asks the reader to fill in the following blank:

KK had a fast ----- system w/horse riders.

The teaching manual mentions "messaging" as the correct answer. What is the difference between "messaging" and "message" here? Is one better over the other?

Solution 1:

The difference is in the emphasis.

A fast message system. [emphasis on the messages]

A fast messaging system. [emphasis on the conveyance of the messages]

In the examples provided in the question, presumably the initial emphasis is on the messages themselves because the idea is newly being introduced. Once the reader is familiar with the idea, the emphasis is on the performance of message conveyance.