If you read the full paragraph of which you quoted the beginning, it pretty much answers your question:
Maps must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structure used in this specification is only intended to describe the required observable semantics of Maps. It is not intended to be a viable implementation model.
So an implementation based on O(n) traversal (actually Ω(n) if we're being precise) would not be spec compliant.
In practice, V8's implementation of Map is hash-based and hence O(1).
Another viable option would be a tree-based implementation, which would give O(log(n)) access times. That difference may or may not matter in practice. Trees may or may not use a little less memory than hash tables, depending on specific implementation details.