It's a frequent topic here. Game media are non-functional data, the FSDG do not require them to allow modification [0]. In this case modification is allowed, but there are no tools for it (README.Debian suggests them not being available to the authors too, so they don't give them special powers over this work, unlike most cases of not providing a source). Software that interprets this data is free. (It's common for fonts and art: many games include bitmaps rendered using Blender-like programs from unpublished data, many fonts have easier to edit sources that are not published.)
The no-selling alone restriction is accepted in other licenses too [1], why wouldn't it be free? [0] https://www.gnu.org/distros/free-system-distribution-guidelines.html#non-functional-data [1] https://www.gnu.org/licenses/license-list.html#SILOFL
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