On pg. 38 of R. Avigdor Miller's Torah Nation we find
Because of their hostility towards Israel, the Samaritans are one of the many testimonials to the truth of Israel's traditions. The fact that they have always had the Five Books of the Torah and the book of Joshua testifies to the age of these books....in view of the fact that Solomon erected the Sanctuary a mere 440 years after Moses (much closer to them than Columbus to us), we may therefore consider ourselves as standing in the days of Solomon holding in our hands the books of Moses and Joshua which are only 440 years old and less...*
It is a common error that the Samaritans possess the book of Joshua (and that it forms the sixth and final book of their Bible). I noticed the same error in Wikipedia's article on the biblical canon, which read "The small community of the remnants of the Samaritans in Palestine includes only their version of the Torah and the book of Joshua in their canon. This grouping is sometimes referred to as the Hexateuch," until I corrected it.
While it certainly is romantic and appealing to think of the Samaritans as possessing a bible containing the Torah and Joshua, it isn't true. The Samaritans do possess a chronicle book called Joshua, but it is medieval, originally written in Arabic and it is not in any way canonical to them.
Moshe Florentin writes, pg. 357-58, in his Late Samaritan Hebrew: A Linguistic Analysis of Its Different Types
In 1908 M. Gaster published a Hebrew version of the Book of Joshua. Because of the impression of (imagined) antiquity, and because of the similarity to the contents of the Book of Joshua in the Masoretic version, Gaster felt that this Hebrew version was the original from which the Arabic Book of Joshua resulted.
But in reality the Hebrew version was translated from the Arabic in the late 19th--or early 20th century!
In fact, why not read an English translation of this book and see for yourself that it is not Sepher Yehoshua at all.
The mistake is understandable in the sense that it is one of those things that gets repeated often, seems more or less plausible, and who would think to check it, unless one happens to be interested in Samaritans (as opposed to, say, using them for polemics).
*Actually, this excerpt is part of a larger passage, as you can see by the .... I feel that this excerpt makes the point I want to address, however I don't want it to seem as if I distorted his words, so I scanned the entire passage which you can view here.