This argument has two pillars; one is that it is difficult to think that this phrase is referring to an imperial succession code, because Emperor Koko's edict can be read as 'the Imperial Throne is the law instituted by Tenchi.'
この説は二つの柱を持ち、一つは光孝天皇の即位詔が、「皇位は天智が定めた法である」と読めることで、この文面では皇位継承法のこととは思われない。