You can only detect gross structures with xrays. It cannot tell you that a material will not be more likely to fail early due to included contaminants, or lack of, or grain structure etc.
By interior composition and distribution I'm not talking about anything as comically stupid as plating like the inside is aluminum.
The surface of a finished part is routinely intentionally quite different from the interior, ie spin casting and case hardening etc. Frequently the performance of the part actually requires that the interior be different from the surface, ie hard shell resilient interior.
You can observe a lot about a finished part in various ways, like just tapping it and observing the sound can be more useful than an xray. But there's a lot you can not know after the fact through observation, except by observation of the eventual failure or not.
For one example, dissimilar materials, either within a casting or even just 2 parts in contact with each other, or a part and a brazing material, can migrate and diffuse into each other over time. Small differences in the initial conditions change how that develops over time, and can result in big changes in the performance of a part later.
You can't examine a finished part to determine that it was fabricated according to the recipe. You can only detect gross problems. You must trust that the supplier and their suppliers all followed the various recipes.
Here's another angle:
They first detected the forged paperwork because the guys on the factory floor observed that the material looked wrong.
So, it's the opposite of "you can't detect the difference". They detected a difference just plain visually.
The counterfeit parts might actually be perfectly sound. We don't know they will fail early, we only know that we can't trust the paperworks claims about how they were produced, where the materials were sourced from, how they were processed etc. Whatever the source and processes actually were, the end result might be inferior, or might be equivalent or even superior. (although detecting pitting they didn't expect does not lean towards the parts being superior)
They are able to observe that there is something different about these parts. They visually looked different enough to raise the question. Yet so far, they haven't been able to say that the parts are actually unsound through any testing or that initial visual observation.
It's not only that a part that looks perfect might not be, it's also true that even when you do detect a difference, it doesn't mean the part is bad.
You can observe a lot, but there is no amount of after-the-fact observation or testing that can replace knowledge of how a thing was produced.