I think the way "retro" is being used here is just as a buzzword for "old" or at best "outdated". That's not really what it means, but, that's fine - the point I'd like to make is: I think we should be careful about drawing a distinction between "Amber has outdated game mechanics" and "Amber as a setting has outdated themes".
As far as I'm concerned the first one is entirely untrue, and where the second is concerned you can say the same about any 70s new wave SF story. Amber's themes have been constantly revisited by other SF authors since. To take Amber out of the time when it was written robs it of its meta-textual relevance, but, it's a story about an infinite multi-verse...I'd argue it updates itself, even unintentionally. Thematically you can't help but update it as you go.
So I suppose in either event, mechanics or setting (at least on an in-play execution model) I'd say there's nothing retro about Amber at all.
//Panjumanju