I’ve worked on two 2013 Cannondale slices. Both came with a seat post nearly 18 inches long.
I cut them to fit their owners needs.
If your LBS told you that then I would find a new LBS fast! There should be a minimum insertion marking on the post. I have had to cut almost every seat post on every TT bike that I’ve owned.
As an LBS employee, I would be wary of any “mechanic” that has never seen a seat post that needs to be cut. Carbon or Aluminum it is a simple and common thing to need to do.
If you want to tackle it yourself, do it with a hacksaw using a vice or a cutting guide to make the cut straight. Be sure to make a new minimum insertion line above the old one by the same amount you cut off - if you take off an inch, move the minimum insertion up and inch. Also, be sure to file the cut afterward to remove any burs or sharp edges that could damage the inside of the seat tube, or cause the seat post to seize.