I'm going to replace the throttle body in.

If you are replacing it with a direct fit part, there should be no problem.

AC Delco is a factory GM part.

That is a good question. Is there anything different?

I don't understand what they want to program.

They may be installing an updated part that requires different programming or they may be correcting a known problem with the original programming. The simple replacement of an identical part doesn't require programming.

Idle relearn is done through normal driving. No programming required.

I am aware of that. I just replaced one on a 6.0 Denali a few days ago.
What we still don't know is if there is an updated flash program that should be installed to correct a problem they found.

Here are the official installation instructions

INSTALLATION PROCEDURE

Install the NEW throttle body gasket.

Install the throttle body.

NOTE: Refer to Fastener Notice in Service Precautions.

Install the throttle body nuts.

Tighten the nuts to 10 N.M (89 lb in).

Install the coolant air bleed hose to the throttle body.
Position the coolant air bleed hose clamp at the throttle body.

Connect the throttle actuator motor electrical connector (2).

IMPORTANT: Verify that the throttle actuator motor harness connector and the connector seal are properly installed and not damaged.

Install the engine sight shield.
Install the air cleaner outlet duct.
Refill the cooling system. Refer to Draining and Filling Cooling System in Cooling System.
Connect a scan tool in order to test for proper throttle-opening and throttle-closing range.
Operate the accelerator pedal and monitor the throttle angles. The accelerator pedal should operate freely, without binding, between a closed throttle, and a wide open throttle (WOT).
Verify that the vehicle meets the following conditions:

The vehicle is not in a reduced engine power mode.
The ignition is ON.
The engine is OFF.

Start the engine.
Inspect for coolant leaks.