No idea why this dealer did, but have seen situations where a dealer/service manager got put on a short leash by Ford for charging questionable work to warranty. Then is gun shy and overreacts the other way.
Personally, I don't like the "fangs" but not the main reason for my suggestion.
IMO, A Mustang should have a corral grill. I believe the 2018-21 Eco grill fits the GT and has the corral. Would be my first choice.