We can argue back and forth on what constitutes "kingly qualities" all day.
What's tough to deny is that, thematically, it doesn't really make sense for anyone with CoC Haki to choose to be or remain a marine.
If another external source to the manga or the manga itself confirms Sengoku has CoC, that theory goes out of the window - for the most part anyway - but until we have that, I'll consider it just one of many errors in the supplemental material.