Whether something should or should not be copyrightable in the moral sense is a very different conversation than in the legal sense and I don't really see why we should be restricted into believing that things that commonly require licenses should always be that way.
The problem is that Luxembourg's copyright regime should be a lot more stricter than, for example, US (this ruling is significantly different from other Luxembourgish rulings). I actually thought that this is a US ruling at first glance.
If the Law governing IP in Luxembourg only protected exact copies, the reasoning wouldn't have had to have been about the pose, it would have been that the painting wasn't and exactly copy of the photograph.
However either way, I see no reason to think about copyright in such a limited way. I don't think many people believe that copyright is only enforceable if two works are exactly the same. Rather I think they're simply discussing how inspiration and imitation is very common in art.