Yeah, it rather seems to me that the metal ki is something the MC recently learned from the Demon God. His body's strength definitely is the result of the "beatings" his master gave him. Which is also why Noh kinda refuses to teach his daughter. It's just not proper in this time period to a. have a woman with a burly body, and b. beat up a girl in general.
Also, it'd be stupid if any random person could learn some kind of ki and suddenly become super strong, without any rules or principles to follow simply in order to stay in control over the new-found skill. If you want to become strong, you also need to know your own physical and mental limits, and build it up slowly but constantly, so your body or mind doesn't get damaged in the process. If the same doesn't apply to ki, then the author should just call it "magic" and start a fantasy story with other nonsensical elements.
Anyway, it was kinda funny to read this chapter during these Corona times. When he said "Does anyone else want to talk about their rights, or is unable to recognize their lack of skill", I could hear all the Karens in the world scream "But mahh riiiights! Mah Frrreeedomss!" in unison in my mind. 😅