I know a lot of time passed by, but I am not sure if the rejection reason for lutris was factually correct. And the uploader didn't follow up with that.
I tested recent deb packages and sources of upstream lutris, including git versions, and there are no non-free components in source, used during build or in binary package.
I also launched and use lutris, and it did not download or used any non free components.
The fact that lutris can be used with non free software, isn't any different than apt or wget can be used with non free software.
It definitively classifies into 'main'. Not 'contrib'.
The only (fixable) issue is that binary package depends on 'unrar', which is non-free, and packages in main afaik can't depend on non-free ones with alternatives. It should instead use Recommends, or use Depends: unrar-free | unrar, or just unrar-free should suffice probably.
PS. It is obviously possible that in 2014, situation was different.
Best regards,
Witold