For M119 engine, aren't cruise control actuator and throttle actuator the same thing?
Not the same thing.
The throttle actuator has combined three separate functions into one part, and if one fuction fails, you have to replace the whole throttle actuator. It is sometimes referred to as the EA / CC / ISC module: Electronic Accelerator/Cruise Cntrl/Idle Speed Cntrl Module.
See this page for a nice description:
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".