Installing IKEA farmhouse sink in 23 inch deep cabinet?
It looks like the sink can be used with a shallower cabinet frame as long as you don't mind more of the sink projecting from the front of the cabinet than what you may have seen when it was displayed in the showroom.

Also, there is a trim board that is usually installed below the sink rim at the front, to hide the bowl. This may not be possible if your cabinet frame does not project out far enough. Your cabinet doors would also need to be installed below the projecting portion of the new sink.

Your "countertop guys" are correct, you should wait until the sink is at the job prior to any alteration of top or cabinet. If you are not handy with cabinet making or carpentry, a good cabinet contractor can likely make this a quality install that looks good too.