So, let's say you're a liberal judge -- or, in this case, a whole bunch of liberal judges ruling en banc -- and you really don't like the Heller decision. However, the author of that decision recently died, so you figure you can tee up the Supreme Court to reverse it in a new precedent. Thus, you decide to issue a ruling completely ignoring the Heller decision, and creating a wholly new standard for what kind of weapons deserve 2nd Amendment protection.
The problem is that the new standards doesn't just ignore Heller. It also directly violates the logic of the prior most-important 2nd Amendment Supreme Court Ruling, United States vs. Miller.
The Miller ruling appears to say that the only weapons the 2nd Amendment protects are those that are suitable for militia service, as for example by being of "ordinary military equipment." What the new 4th Circuit case says is that no weapons are protected if they are "most useful in military service." In other words, the two categories are mutually exclusive: the Supreme Court's standard is exactly the opposite of the 4th Circuit's.
And that's if you throw out the Heller decision entirely, as if it never existed.
However, it does exist.