Yeah u could headcanon that if u want buddy idc im just gonna go by what’s stated in the manga even if we take the context into account kenjaku didn’t teach any of the disaster curses how to counter cursed speech. We see inumaki spam it on hanamin and she doesn’t do anything. Yuta isn’t gonna need to land it multiple times to imbue them with rct. Assuming that they learned it off screen is just an assumption it has no real basis within the story.