I think we can all agree Rav Shlomo Zalman knows the Halacha and is a Yiras Shamayim.
I dont know who put out the Hagaddah since it was put out after his death, but it was likely his sons or grandsons (I am purely guessing here) or perhaps one of his big talmidim with the approval of the sons and grandsons,but Im sure the wet Matza was put in there with someone approval and that it was true