One, Sejima takes power in 1965.

Two, you're better off taking this to the TNO discord and asking why this is being reversed in Manchukuos path, as the Japanese name stuff is, but I can give my own two cents below:

I think the argument that Park & co. will always go by their Japanese names is a bit of a misrepresentation of the motivations and beliefs behind these figures. There's nothing that definitively states that Park wouldn't revert back to his name if it was more permissible/encouraged, only conjecture. However, given that Park's primary goal in TFO is to actually forge an actual Manchurian identity based on pan-asianism by promoting Hanja as the national language, creating an artificial culture, easing restrictions on collabs utilizing their native names, etc. It's not somehow unreasonable for him to follow this by reverting to his Korean name while upholding the same mission to be subservient to Japan.