Is "I hadn't read it yet" correct?

It seems totally fine.

Someone sent her something that she had not read yet. The past perfect refers to the time before that person sent her that text. The sending happened in the past, and the " not having read it" was before that.

If she would have tweeted, as @AkshayPrasath proposes, "Thanks for this.. I haven’t read it yet..", it would mean that she thanks someone for sending her something, but she hasn't found the time yet to read it.

That could be a reasonable thing to say, if someone had given you a book: "Thank you for the book you gave me. Unfortunately, I haven't read it yet."

Yes, it means "at that point in the past, I had not yet read it. However I may have read it since then. "