You missed something.
There is a correction for bedrock motion in Zwally's paper (section 3) that he says is based on a newer model. This is what makes the study significant. If that model is correct (and I'll admit I don't follow all the math in the paper) the other studies, including the GRACE ones, will probably have to correct for it, too.
I find it significant that the usual players are piling on this poor guy like a pack of wolves. I guess this is how "consensus" is formed. It is not a sign of a confident position.
Fun fact: Zwally is the guy who thought in 2007 that the Arctic would be ice-free by 2013.