It's a valid code for the LY7 3.6 ONLY (equipped in the 05-07 STS before they switched to the DI LLT) which is why it's also valid for the gen 1 CTS too.
It it not a valid code for the 4.6.
DTC P0318
Circuit Description
Correct engine misfire detection is dependent on whether or not the vehicle is experiencing a rough road condition. A rough road condition causes unexpected crankshaft fluctuations. In order to detect a rough road condition, the electronic brake control module (EBCM) communicates a wheel speed signal over the serial data circuit to the engine control module (ECM). The wheel speed sensor information allows the ECM to distinguish if the crankshaft speed variations are caused by a rough road condition or are caused by a true misfire. If the ECM does not receive the rough road information, this DTC sets.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0318 Rough Road Sensor Circuit
Conditions for Running the DTC
•There are no EBCM wheel speed sensor DTCs.
•The EBCM is communicating over the serial data circuit for more than 5 seconds.
•DTC P0318 runs continuously once the above conditions are met.
Conditions for Setting the DTC
The ECM does not detect a wheel speed signal from the EBCM for more than 4 seconds.
Action Taken When the DTC Sets
•The control module stores the DTC information into memory when the diagnostic runs and fails.
•The malfunction indicator lamp (MIL) will not illuminate.
•The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Failure Records.
•The driver information center, if equipped, may display a message.
Conditions for Clearing the DTC
•A current DTC Last Test Failed clears when the diagnostic runs and passes.
•A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic.
•Clear the DTC with a scan tool.
Step
Action
Yes
No
1
Did you perform the Diagnostic System Check - Vehicle?
Go to Diagnostic Starting Point - Antilock Brake System
in Antilock Brake System
Go to Diagnostic System Check - Vehicle in Vehicle DTC Information
Basically if its a constant problem the "service writer"(that's what the OP claims to be) who should know better than to do things half ass, needs to look into the EBCM (EBTCM) module replacement. On the 05 4.6 I know for a fact the electronic component is available separate from the hydraulic portion.