Do not plan your travel around what some random TSA Officer might or might not do. Yes, there are occasions when they make the wrong call. But, if it is down to something which matters -- and it would here -- then ask for a supervisor. They have the capacity to pull up their policy as well as the public-facing website.
If it is something odd-ball, it does not hurt to have a printed copy of the TSA website covering the issue.
It is easy to correct an error without getting into the sneering put downs which wind up as problems.