I would bet your triple is now on someone else's bike... happens all the time. It's very weird a shop would even make the suggestion. Were you asking to make the bike lighter or about riding with a double?

If you are fairly strong and lightweight, the compact crank will probably be good for you. It does save a few grams and maybe even an ounce of two. I'm assuming they also changed out the chain and rear derailleur? Have you ridden the bike to see if it shifts well? Not all that easy to change a bike from a triple to double or vice versa. Alot of fine adjustments are needed. The chain has to be shorten and usually the derailleur needs to be replaced.

And yes - you got an orange for an apple. But consider double cranks are uusually cheaper than triple cranks. It's just odd you didn't get the same brand and model. I like Shimano products and especially Ultegra. I'm not a fan of Trek or Trek products. The Bontager Race is not a bad crank. It's just not as good as Shimano Ultegra in my humble opinion.