Anyways, literally the first sentence of the article is false[1]. Self-made firearms in California are required to have a serial number, must be put into the state registry (a somewhat byzantine process), and are illegal to resell[2]. In other words, the gun is either legal OR untraceable. It can't be both at the same time.
[1]: https://oag.ca.gov/system/files/attachments/press-docs/consu...
[2]: For ones manufactured after 2018, your family can't even inherit them when you die— they have to be surrendered to the police for destruction. If you have a relative you really don't like, you should definitely bequeath them your homemade guns— there's a non-zero chance they'll end up in prison for possession.