This is true, the Gerber X3 does contain all the information for a full turnkey assembly (PCB layers + Position + BOM), and it has been supported in Kicad for a while (which is great). Unfortunately, I think X3 came out too late as IPC-2581/DPMX had already gain momentum. Currently, I have not been able to find any assembly house that will quote Gerber X3 files. I think EuroCircuits (who helped define X3) will quote full turnkey, but I don’t believe they ship to the USA.