I like seeing the occasional effect of running into the Envious card. Makes you think twice about, for example, buying a Cursed Village. It is a nice subtle touch, to have all your golds and silvers suddenly worth only one coin, for that hand. But it seems odd to leave out Platinum from this penalty.
Intentional? Merely an oversight???
Presumably for the same reasons other non-Prosperity cards such as Bandit Fort and Noble Brigand "exclude" Platinum. Probably because, unlike Gold and Silver which are Basic cards and always in the Supply, Platinum is only in the Supply under certain conditions (i.e., randomly based on the number of Prosperity cards in the Kingdom). I don't think any card from a certain set refers to cards from different sets. For example, no cards outside of those from Dark Ages specifically mentions Ruins and Shelters. So it seems pretty consistent for cards from Nocturne to not specify Platinum.