Time to sperg about this: giving the Reimu a ko is going to *encourage* it to sing at first. Then the fake ko's responses are going to prompt lots of "Why would you say thaaaat!" Things are not going to get quieter for a while.
Also, "2 months later." Sounds like it takes a while for the lesson to set in.
Perfect for a lazy owner who doesn't want to discipline their yukkuri themselves. I guess.