No one is trying to pass aluminum or steel as titanium.
It's pretty straightforward to pass one titanium alloy as another, or claim provenance or material properties it doesn't have. I have two indistinguishable scrap pieces on my desk right now, one Grade 5 and one Grade 2. It's also possible to pass a billet or sheet of alloy with defects or poor quality control, voids, or inclusions. "Titanium" is a broad class of materials that are indistinguishable without exotic tools like XRF guns, or, in this case, a well documented and trusted supply chain.
Alloy substitutions and similar fraud happen all the time. It can even be the same alloy but have issues in post treatment and not meet spec. Here's a case where a NASA supplier was committing this fraud for over 20 years. It included fraudulent documentation, but the material itself was not up to spec:
https://www.sciencealert.com/a-supplier-was-delivering-fault...