According to his own website, GRRM hasn't even finished it yet. I'm not believing anything about this book until I can buy it.
My understanding was that there was a chance it might make things worse. If the mud didn't actually slow the leak, but was pushed out as fast as it was applied, there was the fear it might further damage the already broken valve. So, rather than a partially open valve somewhat checking the flow of oil, you'd have a fully open pipe.
Except, at least with the deal we got from So Cal Edison, we give them the right to shut off our air conditioner in exchange for a discount on our summer electric bill. I don't recall exactly how much of a discount on the energy they gave us, but considering that they never once actually killed our air con during the summer, I have no complaints whatsoever.