"provides an exception under IRC 3121(b)(6)(C)"

Per Cornell:

"(6) service performed in the employ of the United States or any instrumentality of the United States if such service is performed—

...

(C) by any individual as an employee serving on a temporary basis in case of fire, storm, earthquake, flood, or other similar emergency;"

"I do have a client that worked as a volunteer firefighter during an emergency...she was paid via a 1099-NEC"

Well, that seems settled, then.