Is this worth $1900?

I don’t think it is a 2018 model. It looks like Colnago renamed their CX bike to the Prestige in ’18. Probably a year or two older. Might have been bought on clearance in ’18.

There are some concerns I have. The bike has a couple things that are dated. It uses post mount disc brakes. It has quick release (QR) skewers for the wheels. Neither of these are used in current road bicycles. If you have problems with parts you might need to find old stock for the shifters and possibly the brakes. The wheels are not really a problem parts wise. QR wheels are still made, they don’t perform the best with hydraulic disc brakes. They don’t have the repeatably in position that disc brakes need. You may need to play with tension or centering to get the brakes not to rub or ping. 

Modern road bikes use thru axles and flat mount brakes. Just a year or two newer and the bikes usually have those features.

All that being said for a carbon Di2 11 speed bike the price seems okay. Though better pictures would be helpful. The picture you submitted does not show the drivetrain.

Colnago uses their own sizes. 42 is not 42cm it is closer to a 48cm-50cm frame.

Colnago is an expensive brand but i would buy a newer bike with later groupset. For example with a 12 speed shimano 105 di2 (or mechanical). Price seems a little steep for me at least.