Oh I saw this after JKR had written those tweets!
It really is a steaming pile of horseshit isn't it? 'Luna' is a boy who likes pink and My Little Pony. None of that means that Luna 'might' be trans as the author seems to suggest. I don't even get what she is saying - she seems to think she is making some clever, profound point about gender stereotypes whilst totally enforcing those stereotypes by suggesting Luna's love of dresses might mean she is actually a girl in a boys body? 
And what's with all the 'you hurt my Luna' - no you hurt Luna, Sarah, by planting the seed in his head that his love of pink means that there might be something 'wrong' with him that needs 'correcting'.
It's trying so hard to be emotionally manipulative but the argument is so crap it doesn't even achieve that. If JKR ever did see that letter, I really don't think it would trigger the epiphany that Sarah seems to think it would.