You are correct; only the poster can edit and change their own original post.
If you use the quote feature, you can go into the quote of the other person’s post and rewrite it as much as you want. That’s why it is not really useful as a “quote for posterity”.
As far as I know, no one has used that technological feature to intentionally distort a quote. A couple people did it very openly to demonstrate how it could be done.
When someone asked if anyone had used the quote feature to distort a quoted post of another poster, Trubandloki volunteered that I had, which is a lie.