Why is "DMV" different from a language point of view in Linguistics?

A simple way of thinking about a DMV is as a kind of tree-like representation of a parsed phrase. Here is an example taken from the Wikipedia page for "Dependency Grammar."

This is an "unordered tree" of the sentence:

That thing I will never forget.

However, you would get the same tree if you were parsing the sentence:

I will never forget that thing.

So comparing those two sentences, one might say:

They may be equivalent as far as the DMV is concerned, but they're not equivalent from the point of view of language.

I'm not sure whether or not this example will help you understand the comment addressed to you without knowing more detail.