She could have written it
Written by Thérèse
(3/15/2013 6:30 p.m.)
in consequence of the missive, explanations, penned by Stephanie
provided she would have wanted Harriet to know her engagement to Mr Knightley; what I don't think she wanted yet.
I will go further : nobody had to know about it before her father, and it was days before she was able to tell him; just after they went to Randalls and it was further disclosed.
So, I don't think the letter would confess the engagement, nevertheless I find it a very good idea that she might have disclosed Mr Knightley's love for another woman than Harriet. Having told it her just when coming back from London, Harriet could believe him in love there; although she maight, too, have guessed Emma's love for Mr Knightley, when thinking of their last meeting.