I noticed you created a game with this asset and submit to greenlight. When you submit a game to greenlight other people cannot use the same asset. Valve delete games with assets. from other games.
Your game link:
Guess someone is using them in that game, but if that's about steam's policies then idk. If they are blocking you it seems like a reasonable anti-theft rule that just unfortunately doesn't account for indie games with freely-released graphics. Maybe someone will be able to sort it out. Don't know much else about it though so good luck.