It's not that the titanium metal is fake, it's probably real. By "fake" what they mean is that the part made with the titanium isn't what the documentation for it claims. It may have been made by a different company than what the documentation says or it may not have been tested in the way that the documentation says.

Every last piece, down to the tiniest screw, in a commercial airplane has a paper trail and for these parts someone is saying that the paper trail has been falsified. That's the "fake" part.

Still titanium, but likely sourced differently and not batch tested to verify exact mechanical properties. Only time you'd tell the difference is when it fails prematurely.

It’s not like they swapped titanium for balsa wood. The origin docs were falsified or missing, which could mean anything from they weren’t the right purity but were shipped anyway to they were imported from Russia and illegally bypassing sanctions.

This is a supply chain integrity thing. They're not alleging that the titanium wasn't actually titanium (which as you said, would be noticed immediately).

How the heck does one not notice that a sheet of titanium is fake?

Contains a certain level of impurities that doesn't meet with guarantees?

Right, but they were likely so cheap that it didn’t matter.