Owning a ghost gun is a crime, right?

(Ignoring the fact that “ghost gun” is a meaningless and intentionally emotionally charged term)

In New York, yes. In the vast majority of the US, no. It’s illegal to file the serial number off an existing firearm, but 100% legal in most states to manufacture your own unserialized firearms for personal use. Just cannot be sold/transferred.

I’d note the article you linked says nothing about how many of those are actually 3D printed, it is infinitely easier to deface the serial number on an existing firearm than it is to 3D print one.