I don't know what flag they were trying to show, but the answer as to why they were carrying it may be revealed in this photo ... they certainly do not appear to be the most "authentic" reenactors on the field (putting it mildly). As a reenactor since 1973 (but rarely anymore), I can say that the reenacting hobby seems to go through a continuous cycle of re-inventing the wheel. Even back in the late 1970s there were groups that based their reproduction uniforms and flags on originals, and attempted to carry flags appropriate to the battles they were portraying. But that knowledge seems to be periodically forgotten and then "rediscovered" all over again, but in the meantime you see "uniforms" and flags like this.