[mechanicum]

I don’t think that necessarily follows. The age of the surviving fragments today isn’t the whole story.

We could presumably infer it still wasn’t “missing” as recently as a thousand years ago from later sources referring to it, even if the specific text (or oral tradition) those authors knew of hasn’t survived.

Like how we know about some of now lost Greek plays, originally written in the 5th century BC, because they were still being performed in Imperial Rome and writers of that time described them, even the details of how they were staged.