True -- but each of those scribes wrote only short segments, almost all of which are confined to the opening pages.
Rigdon's written contribution is much more than half. I'd have to count the number of lines in the manuscript(s)
to be certain, but my recollection is something like 90% is in his handwriting and a few lines are in Smith's; and a
few markings/notations in the accompanying Bible are in Smith's hand.
I have dismissed from my answer those other people --- scribes, as I think you correctly identify them. Perhaps I
ought to factor Cowdery back into the equation, as possibly having been more than a scribe. But my current
thoughts weigh against that. I do not see Cowdery as having been a theologian nor a homilist, nor much of an
exhorter. He did not spend the better part of his life receiving revelations and preaching doctrine. I am more
inclined to dismiss his contributions as merely scribal ones.
But in the case of Rigdon, I see things differently. I hold open the possibility that he was a mere scribe at about a
10% probability -- and that he was a textual compiler/creator at about 90%. I could've course be wrong, but I
would have to see a great deal of opposing evidence, before those percentages would change much in my mind.
So, I go back to my earlier statement, that I am very much inclined to see the compilers/creators of Book of Mormon
text as being the same as those who produced the Moses and Enoch passages -- and probably that each text was
produced in a similar manner as the others. So far, I exclude the Book of Abraham -- and I have never seen a
word from the Book of Joseph (reportedly found with the mummies).
Is it possible that I am wrong? Yes, that is possible -- but until convinced otherwise, that remains my working view
of the past. I would be happy to see primary source material brought to light in a non-biased way, which could
help me fill in the blanks in my own scant knowledge.
The following is from Scott H. Faulring, Kent P. Jackson, and Robert J. Matthews, Joseph Smith's New Translation of the Bible (Provo, UT: Religious Studies Center BYU, 2004), 46.
His [Rigdon's] labor as scribe for the JST dictation extended from December 1830 to 24 March 1832, and over half of its pages are in his handwriting. In addition, he was the primary scribe for most of the changes that the prophet made after the original dictation, and thus most of the insertions written on small pieces of paper and pinned to the manuscript are in his handwriting.
The following is a rough estimate of number of pages each scribe contributed to the Bible Revision MSS:
Old Testament & New Testament MSS #1
Oliver Cowdery 10
John Whitmer 4
Emma Smith 4
Sidney Rigdon 109
Old Testament & New Testament MSS #2
John Whitmer 83
Sidney Rigdon 129
F. G. Williams 63
Joseph Smith 4