This comes from 鈴木為次郎『囲碁独習』日本棋院 1931 (“Self-studying Go” by Tamejiro Suzuki for the newly established/merged Nihon kiin at 1924)
The reason that amateur (素人) rank only goes down to 15k seems to be that the beginner(初心者) was given 25 handicaps(15k), with some big gaps - 20 handicaps(14k), 17 handicaps(13k), 13 handicaps(12k), till 11 handicaps(11k), and then jumped to 9 to 2 handicaps(10k~3k), and josen and tagaisen (2k~1k), where komi was yet to be introduced.
And dan players and Nihon kiin players had their own smaller handicaps 手合割, all dan players were considered “professionals” (專門家)
They seemed to already recognize that lower-ranked players would have trouble utilizing handicap stones.