The information is good but the final part is not correct or at least inaccurate. You do not need to speak to the Horse Guai in Chapter 5 and you do not need to finish the whole Chapter to obtain the Dark Thunder Transformation. All you need to do is to win the boss battle against the Ying Yang Fish in the palace. After that fight (in which you also have to fight the horse guai in a sort of spirit form) you can go to the location of the cart and you will directly have the option to puဣll forcefully and will obtain the transformation.