Wood already shrunk because it's been up for years , key words in your statement shrink a 1/64" , shrink means shrink.
Don't know how tight mix will be, don't assume installer will mix tight even if he does still 1/64"???
Sure if you take it out of context and remove the "wouldn't even shrink", it takes a new meaning...........
Think of it this way. When a 9' tall basement wall is poured, how much do you think it shrinks vertically? Not enough for crude measuring tools to detect. Why would such a small profile of concrete (1.5") be prone to such massive shrinkage? The answer is it won't.