A couple thoughts after viewing:
I don't think the Witnesses are/were claiming to testify of the book's historicity. And I don't think apologists are making that claim either. In fact, the naiveté of both Joseph and the Witnesses further the apologist's goals.
The LDS apologist cares about the witnesses for only one reason - they testify to the physicality of the golden plates. The witnesses testify that the plates were real physical objects (or so the apologists claim).
Once you have physical gold plates, everything else follows as a consequence. Joseph couldn't have written the book, so it must have been written by someone else. Joseph couldn't have made the plates, and Joseph couldn't have known ancient history, so the plates must be an authentic ancient artifact. And that implies that the book is historical.
It's a giant causal chain that begins with the witnesses testifying that the plates were real and ends with the book being historical.
I only know this because DCP has been harping on it for years. For him the plates are the foundational artifact that require convoluted explanations to argue that the plates were fake or didn't exist.
So to go back to your title - the Witnesses were neither witnesses to history or faith, they were witnesses to plates.
The 8 witnesses testimony in particular drives this home. They leave behind all of the miraculous elements of the 3 witness testimony:
Be it known unto all nations, kindreds, tongues, and people, unto whom this work shall come: That Joseph Smith, Jun., the translator of this work, has shown unto us the plates of which hath been spoken, which have the appearance of gold; and as many of the leaves as the said Smith has translated we did handle with our hands; and we also saw the engravings thereon, all of which has the appearance of ancient work, and of curious workmanship. And this we bear record with words of soberness, that the said Smith has shown unto us, for we have seen and hefted, and know of a surety that the said Smith has got the plates of which we have spoken.