There's no reference to a "ghost gun" in the LA Times text. It's only in the headline. In the text, the weapon is referred to as an AR-15. SFGATE, though, quotes cops as saying "AR-styled assault rifle also known as a ghost gun.”

The "ghost gun" thing refers to guns without serial numbers. For an AR-15, that usually means something assembled from an "80% lower receiver", a part which exists purely to evade US gun laws. [1] It's not quite complete; it takes some final machining. This can be done by hand with modest machine shop tools, or there's a special-purpose 3-axis CNC mill to do it for $4000, available from Ghost Gunner.[2] (They used to sell a simpler device for $1200 which was only useful for the single task of finishing "80% lower receivers", but the current $4000 unit is halfway respectable as a desktop CNC mill. It understands a subset of G-Code. Nobody seems to use it for general CNC work, though. A good desktop CNC mill for $4000 would be useful, but they'd need to get serious about cooling and chip removal.)

[1] https://gundigest.com/more/how-to/gunsmithing/building-a-gho...

[2] https://ghostgunner.net/