This is not very handy.

Maybe he could give the engine along with the data, but make the user aware, in the license, that
he pays only for the quest, and that
the engine, already included for facilitation, can be downloaded anyway on solarus-games.org.
Imagine he wants to sell an Android app. This would be very hard for a simple user to downloaded the quest on Google Play and the, go to solarus-games.org and downloaded the engine !
Perhaps, we should recommand to always display the Solarus logo in the introduction, and add a line to specify its license there ?