The problem with art is there is often no licence, or people try to apply software licences to it.
For example I run icewm.themes.org. Most of the stuff here has no licence at all. Is it freely distributable or just from the web page? Can you modify themes, port them to other WMs or use their graphics in your own themes? Is it owned by the poster or VA?
I've been told will be addressed in the next version of t.o (the poster will be able to specify a licence). But since few are explicitly stating their licence what are we to assume? It's not our job to force people to supply a licence.
Another example, the Propaganda tiles are GPL (although no licence is mentioned for the post t.o page, and the link to the licence on t.o is dead). This no doubt means that you can distribute them and modify them as you please, as long as you supply the source. But what is the source, many of these tiles have original images that were modified and made seamless, doesn't GPL imply that these source images should be available too.
Point is, artists need to understand licencing issues. In your case they have and seem to have made their wishes explicit. The best you can do is clarify what they will allow you to do with work derived from their models. But at least they have given you a starting point and I would recommend assuming that non-commercial means just that unless they tell you otherwise.
This does mean that it can't be distruted on a commercial distro. I would suggest splitting you game into the free section and a non commercial section making it clear the licencing requirements of each. Of course, contact the authors and see what they want done first.
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