Dbrand is lucky they haven’t been sued by the board manufacturers for creating an unlicensed derivative work (which is what the case art is, just as the photo of a sculpture, even stylized, has been deemed derivative - especially when the reproduction is intended to represent the original).

I think "lucky" is an overstatement but this is an interesting point and could be the knockoff company's defense ie:

we couldn't have infringed their IP because they don't own the IP

But I think this line of defense would open both case makers up to a suit from the phone manufacturers. Dbrand is well familiar with IP issues with hardware OEMs though so I don't know...