Thewissen's information was published in 2001. Look up "innominate bone".
Ah. So I take it you're suggesting that the pelvic girdle was not in fact written up until 2002, and so Batten didn't have access to the information when he wrote his addendum. You may be right. Thewissen said he "described" the pelvic girdle in 1996, and he has a paper on Ambulocetus published in 1996. Was it in there? I don't know, and I'm not going to pay $50 for something in a language I can't read to try to find out.
I suppose it's also possible that the initial recovery of the innominates happened in 1996 but that excavations continued after that, and the 2000 article (that's when it was written) was a summing up of all the evidence collected to that date. I don't know if papers like that always deal only with the most recent findings; perhaps one of the scientists here can shed light on the question.
Nevertheless, even if the information first appeared in 2002, Batten has had six years to correct his original statement or update it with another addendum. The fact that he hasn't suggests to me that presenting all the evidence is not high on his list of priorities.