The issue is not that the parts aren't titanium, its that there isn't a paper trail documenting the titanium.
This is an issue, because improperly forged titanium can have issues that makes it unsuitably weak for its intended purpose. Having documentation showing where the materials came from, when it was inspected for defects and when it was manufactured is critical for safety.
United flight 232 had an engine explode in part due to defective titanium. This is a real safety concern.
Though the headline says boeing, the article mentions these undocumented parts being found in airbus planes as well. Its an industry problem, not a Boeing specific one.