Oenothèque is the French version of the Italian enoteca and German vinothek. Essentially, all these words just refer to a place to store wine or a wine library, though enoteca in particular often means a local wine shop.
The Moët & Chandon Brut Champagne Cuvée Dom Pérignon Oenothèque is a wine that’s late-disgorged after spending extra years in the cellar. In this case, “Oenothèque” refers to the place where the wine was stored for the extra time.