I can understand the impulse of being so frustrated, so you just write a post or comment to get it out. It is satisfying to see your argument laid out, to review and refine it. Share it to people who's opinion you respect and see what they think. Fix it to address their critiques. Then what? Might as well post it. There's no cost.

If they think this will spur a mass movement then they are too disconnected to take advice from. But it could be for internal propaganda/discussion, to serve the ego of the author, to get their ideas into the historical record, to influence some small specific group externally.

The letter was discussed by an IRGC spokesperson and the letter was distributed through Iran media outlets. I included links to 3 different Iranian media outlets in the OP.

Do they expect Americans to read 25 pages? Idk. Probably not.

This is part of a sentence from the letter "the irreparable and gaping cracks it has created in the edifice of U.S. global hegemony". There's at least 3 words there that the average American might not understand.

Maybe Iran is overestimating the capabilities of the average American or maybe the letter is just a gesture of good will, a gesture that will look good to non-Americans.