Sadly, Guzzi has made worse errors, like the 8 valve engines that ate their tappets. And that seems like a matter of getting the valve spring rates wrong. But I don't think we will ever know for sure what they messed up to cause that issue. Some issues with Guzzi's are quality control issues, where a certain number of them are just poorly made, perhaps using poor quality parts, that didn't get caught in QC. But sadly other issues are basic design issues. So, an aluminum oil pump gear is not surprising. They may have simply had some alumnum oil pump gears laying around, and while the engineer may have spec'd steel, if they had aluminum gears laying around leftover from something else they may have just decided to use them.