I'll start by saying that I know very little about Campy. All of my wrenching is on Shimano and SRAM.

That said, I can't understand why you would need a new drivetrain. You should be able to buy a compact crankset that's compatible with your current bottom bracket, adjust your front derailleur accordingly, and get on with riding.

Can you ask them for a better explanation? Maybe even call another shop?

I'm as confused by their explanation as you are.