I think the line changed to make it seem like Tywin was the mastermind, as ever.
Jamie has already had some revelation type moment on screen, even if it took him longer in the books.
So they don't wish to reset that in the viewers mind by throwing in a line which would suggest that he masterminded the entire affair, the entire red wedding.
The scene was good, it was well done, the Arya arrival was timely. Things are being tied up nicely. Was it Roose Bolton stabbed him in the book, I don't recall?
I assume Blackfish had to be elsewhere so they can show his character escaping, whilst everyone else is hostage or dead.