If you read the article the letter is found in you’ll not that while it may or may not be a teaching (we don’t have the original letter that this one is responding to for context), this letter is not the only evidence.
The above is a part of the whole quote, but the most relevant, refer to the original in the link below for the full quote.
The letter then follows the above quote. The letter affirms that the above teachings are in fact regarded as the assertion maintained by the LDS church. Again, see the following link for full context.
utlm.org/onlineresources/cumorah.htmThese three books, are standards in the LDS library. They are often quoted by teachers in their lesson plans, and by members who give talks from the pulpit. I have heard GAs quote from and refer to them during general conferences. They are not just some book that some “average mormon” wrote. These books are often recommended to new members to help indoctrinate them with proper LDS teachings. Which is why most people consider them to be accurate in their transmission of the LDS Church’s position.
That said, it is now claimed that three years after the issuance of this letter another was issued correcting it. This second letter has yet to be produced, so until it is, the validity of this claim is unverifiable. However even if the second letter denies that the LDS church has long maintained that the hill is in NY (and thus the BoM set in NY) then it must be reconciled against a long standing tradition of “apostles,” and “prophets” teaching that it is.