If Fukai-no-Joten/ Fukaijoten is not formulated to rule imperial succession, but to advocate the ideal position of the emperors, the edicts of the former emperors as well as the edicts of these emperors in the Heian period would make sense.
不改常典が継承方法ではなく天皇のあり方を規定したものとするなら、前代の詔もこれら平安時代の詔も意味が通る。