Stephen King deeply admires her as a writer. He's written many reviews of her books and championed her work for years. He has a strong and unusual (though not unique) reputation as a genre writers whose writing is taken seriously by critics and the public alike. In his reviews, which are written for major, serious publications, he has made it clear that she belongs in this category as well.
This explains his following of her on Twitter.
Stephen King has made numerous feminist statements in the past (though this has been an evolution). He also tends to stand up for those who have been marginalized, bullied, or cast aside in society.
I believe his first tweet is about the former. I believe his second tweet is about the latter.