Samsung did some kind of testing. I am gonna take a guess.
Samsung handed out a limited number of prototypes for internal testing. They got passed around to various testers, and the units survived the use/abuse of several sets of hands and pockets. But these units are the survivors. They survived for various reasons, but perhaps one of them is that they happened to have been "broken in," properly. Perhaps they had gone through the robot folding machine for a few thousand spin cycles, even.
There's enough play in the hinge that humans folding the phone produces different strains, and the break-in is not consistent. The laminated screens takes a set (or a number of sets) out of alignment, producing a variety of delamination failures.
Samsung focuses on the number of fold cycles. So of course a brand new phone will last longer than a used phone.... nope. If the phone has been carefully/precisely folded a few hundred times, it perhaps will now take a lot more abuse before failure than a brand new phone that is so handled.
There is also perhaps a time variable. The initial prototypes were potentially handled right away, relatively soon after manufacture. Then they made a bunch that sat in a box for weeks before the break in began, and the adhesives and polymers had already set in a way that interfered with a proper break in.
This kind of thing, I bet it requires some sort of proper break in, and furthermore it may need to be performed within a certain time frame after the screen is manufactured.