I'm not understanding your point. The dictionary entry shows that Marshall is a formal rank with various military and civilian meanings. Marshall means a paticular type of military officer, police officer, or parade officer, basically. It's very narrow in it's own way, and it suggests the PC works for someone directly.
Warlord can refer to anyone who excercises military power, be it a Noble pacifiying a region on behalf of their liege, the leader of a Hobgoblin Warband, a rogue mercenary captain, or even a Bandit King. I'll admit it's a somewhat fanciful name for PC's, who presumably don't actually command an army, but Warlord still comes across as less limiting to me personally than Marshall.