You're confusing the definition of a singular thing with the group of things.

For example, apples are a luxury in that nobody needs them to live (and the same can be said of any single food source - and therefore whether or not we consider something a luxury comes down to its availability and cost, nothing to do with champagne being more or less "needed" than beer), but they are also part of a bigger group we call "food" that definitely is not a luxury.