Yeah, because nobody can ever have memorized anything. Anyone who's ever recited anything word for word had to either have a book with it in front of them or some magic 'seer stones' and a hat.

Meanwhile, back in reality, Cantor Gad Lewis -- who is
blind -- recites everything from memory, including texts which are longer than the Isaiah portion you've highlighted, such as Psalm 151 (a Psalm found in the LXX, but not the Masoretic text):
And, y'know, leads the choir in entire services like the midnight praises (hours of Psalms, canticles, and other prayers, again obviously all from memory):
I don't think it's a miracle that this happens, by the way. People can memorize quite a lot when they come from cultures where it is valued that they do so (as has been the case for most of human history), or if they have other motivations for doing so (personal piety, etc.) -- all without the aid of magic spectacles, rocks, or hats. This particular cantor is far from the only person to have shown such skill despite his inability to read. The greatest Coptic cantor of modern times, Mikhail Girgis El Batanouny (1873-1957), was also blind, and trained many generations of cantors in the traditional melodies of Coptic chant, either personally or through the treasury of recordings that was left to the Library of Congress upon his passing.
Given examples such as these, it's basically impossible for me to see Joseph Smith's insertion of portions of Isaiah into his own book as any kind of miracle or confirmation of some supposed truth found in the BOM. It's nothing of the kind.
These are all your opinions as a Mormon, and exactly the opinions we would expect you to have. That's okay. I just don't agree with any of them and find them to be wrong, as Christ's incarnation, life, preaching, death, and resurrection are not things to be 'shared' with weak replacement religions borne of the fever dreams or machinations of self-proclaimed prophets who bring forth other gospels which they seek to entice Christians to follow other gods of the false prophets' making, as is definitely the case with Mormonism, and will always be the case no matter how much you repeat "Jesus Christ", "Love", "God", "Bible", etc. These are not magic words that you can just say and we will fall all over ourselves to follow Joseph Smith as surely as he followed the angels he imagined himself to have received revelation from. Thankfully we do not live in as religiously and culturally isolated a time as Joseph Smith and those around him did. We can and ought to compare what we he brought to what we have been given by our mothers and fathers in the faith, and when that is done,
100% of the time the spiritually, Biblically, historically, patristically, and prayerfully well-rooted Christian of every particular type finds much at variance with our religion in the message of Joseph Smith and the other Mormon leaders, and it is not due to any failing on the part of any of these diverse sources. It is because at its root Joseph Smith's message and hence his religion comes from some place other than the revelation of the Father, Son, and Holy Spirit, the One God we bow down to in worship. It is Joseph's private revelation from some beings he claimed were "God the Father and Jesus" (at least in some versions of his story), which he submitted to no established church of any kind (in contradiction of the example of St. Paul, who went to meet the disciples, as recorded in Acts 21), and on the word of that supposed vision he established his own 'church' by which his doctrine -- not the doctrine of Christianity -- would be preached in lieu of the faith delivered once for all to the saints.
There's nothing for the Christian to rejoice in in that. It's really quite a tragedy, when we consider the ~16 million who are on the rolls of the LDS religion who have been lied to regarding exactly what kind of organization they are involved in. They are told it is not only a church when it isn't that, but even that it is
the Church established by Jesus Christ, when it clearly isn't that either. Lord have mercy.