If you consider the problem Tree traversal, then the correct answer is indeed:
Traversal requires $\Omega$(n) time, since it must visit every node.
We are here talking about requirement and lower bound, as in "what is necessary".
If you want to talk about common algorithms for tree traversal, then it is true that they also have a complexity $O(n)$, which means that it is an upper bound in "what we can do".
However, since your question, the wikipedia page was modified and the current sentence does not really make any sense (basically "Tree traversal is $O(n)$. Since it is also $O(n)$, it is optimal.").