When the System was still around, it did mention something about what Beru was talking about. There could be only one occupying the position of the general of the army.
So, Soldiers that recently reached the Marshal grade must establish a hierarchy with the one before it – didn't the System say something like that?
If he were to assume the general of the army was the Grand-Marshal, then indeed, Beru, as the recently-promoted Marshal, possessed the qualification to challenge Bellion.
Perhaps befitting an ant-type monster familiar with living in a large community, Beru wanted to establish the proper pecking order between the Marshals, it seemed.
'Marshals going at it, huh...?'
Just to make sure, Jin-Woo shifted his gaze over to his other Marshal, Igrit, but the black knight politely declined the proposition. It seemed that he wasn't interested in establishing the hierarchy between the Marshals at all.