He expressed his view that 'only the postscript (that provoked Ieyasu) may be a forgery from a later period' considering that 'there are many manuscripts with only a few differences, but they are generally reliable historical materials' because in the letter remonstrated with Ieyasu's conquest of Uesugi by three magistrates and arbitration office in the Toyotomi regime, it is written that 'your anger is quite reasonable since Naoe's conduct did not reach you this time', the content of the letter from Kagekatsu UESUGI to senior vassals was remarkably similar to that of Naoejo, among other things.
豊臣政権の三奉行・三中老が家康の上杉征伐を諌めた連書状に「今度直江所行相届かざる儀、御立腹もっともに存じ候」とあることや、上杉景勝が重臣にあてた書状の内容が直江状に酷似している点などから、「多数の伝本があり、少なからず異同も見られるが、全体としては信用できる史料」とし、「（家康を挑発した）追而書だけは後世の偽作の可能性がある」との見方を示した。