It would be interesting to read the source material, or some confirmation of the above statement.
I am guessing that this is referring to a comment that old zinc castings will disintegrate into a powder. This was a known problem in some old diecast railroad models. It turned out that the problem was caused by impurities in the alloy which caused internal corrosion. It has been found that more accurately mixed metals do not have this problem.
It was also found that moulds and melting apparatus should be different for lead casting and zinc casting. The zinc somehow contaminates the pieces and will not cast good lead bullets afterward. This is probably not a serious problem with cannon size shot.