They didn't have
literally no involvement—just more minor supporting involvement and oversight. There are 13 people from IS credited (plus 4 from NPD), ranging from Director (Kusakihara), Producer (Higuchi), programming support, character design, illustration, sound direction, and music. From
this interview, translated from Famitsu:
But yes, KT's Three Kingdoms team did the vast majority of development on Three Houses. (Which is why it has so many visual and technical flaws, lol.)
Kusakihara admitted that it was his direction that led to the game ballooning in size—so one of the game's biggest issues (over-ambition) can be squarely attributed to him, and by extension IS: