As Ashborn was a Ruler before he left them after the killing of the Absolute Being, it seems that his name as the Shadow Monarch is only in name and not in actuality. He was a Ruler and never seemed to become a Monarch in any way other than name. Please keep in mind that this may be incorrect as it was a while since I read the novel.

i read the light novel again and saw that the receptacle of one of the rulers, christopher reed when the monarchs went to kill him, he used this skill so i didn’t understand why antares had found it strange that jin woo had this ability too

It Might Just be That Antares Didn't Know Rulers Were Capable of Spiritual Body Manifests, or he Wasn't Aware That it Was Something They Would Have to Learn And Not just Have Like The Monarchs do. Christopher Reed's Manifest Might Also be a Pruduct of The Ruler Combined With His Soul in Some Sense, Where it's a Magical Trait if That Makes Any Sense.

The Hidden Form meaning is the source of the power .Example :Thomas Andre Hidden Form is One of the seven rulers beside brightness fragment ,because thomas andre just borrow the ruler power but technically he still human ,not higher being like ruler and monarch .but those who not vessel is not have hidden form .example jinwoo and the other monarch ,because ashborn itself gave the entire life and power to jinwoo .so basically jinwoo is "ashborn himself" with human soul.and the giant black megazord mostly is spiritual manifestasion / true form of jinwoo as new shadow monarch ,just like when other monarch reveal their true form (like beast monarch with four of his leg before get killed by jinwoo) ,but not spiritual manifestation like nation level hunter .