The short of it is that the Chil Sung and Yungno forms were created by Hwang Kee based off of his interpretations of various CMA and MYDTJ. These forms are considered the essence of SBD is supposed to be.
Excellent point - the story is that both sets of forms were "translated" from the MDTJ....but as I'm sure you know, they are definately not in there. He may have taken some of the movements described and reformulated them into new forms, but there are not 13 forms in the Kwon Bup Bo chapter.
Most of the chinese influence came from HK's time working on the Chinese railroad and learning the southern chinese styles.