To my understanding, you were right; the agent wrong. (Are you *sure* you were talking to a TSA agent, BTW?) While the TSA controls the screening, the airlines control the lines leading up to them. Hence, the agents manning those lines are frequently contractors working for the airport/airline, and not actually TSA. In airports where there is a priority line, it is up the airlines to decide who can use them. Once you reach the screener, though, the TSA can do what they please.

FWIW, you'd probably get a more expert answer to this question in the TS/S forum.