Alas! java.util.LinkedList is not a data structure by itself.
Does this mean that an array isn't a data structure either? or binary tree (because that's used to implement a SortedMap or a Heap)?
Ask yourself: "Is it data? is it structured?" if the answer to both of these is "yes", then, well, you have a data structure of some sort. Give the data structure Wikipedia page a read.
That a given structure is used to implement other things doesn't mean that it isn't a structure itself.
Arrays are used to implement flexible arrays. Flexible arrays are used to implement the hash table part of a map (or dictionary if your language uses that jargon instead). Linked lists may be thought of (kind of wrong, but bear with me) as specialized form of a tree (one branch only). Trees are used to implement... well, heaps, sorted maps, b trees (which are often found in indexes in databases and file systems). The list can can go on and on and on (and that isn't a complete list).
Some of these things that I mentioned are fuzzy in their implementation. One can implement a Map with a hash table (backed by an array) - a HashMap, a red-black tree (a particular implementation of a balanced binary tree - just be glad its not an avl tree) as a SortedMap, or even linked lists as a ConcurrentSkipListMap. The key thing is how they behave - all of these implementations implement a set of the same methods guaranteeing they all have a get(K) and put(K, V) (along with some other things).
The linked list (concrete implementation) is a particular way of implementing a List (an abstract data type). You could also do it with a flexible array (another concrete implementation).
That one structure is used as the foundation for another structure does not make it less of a structure. The key point in the data structure is that there is data, and it is structured in some way so you know where to look for its components.