i dont get it, but i still try to comprehend the situation. i think what kazuya wants is so mizuhara don't have any regret. imagine you lie to your closest relative, the lie continues to their grave when all they want is the best for you. sure, they're happy when they're alive, and them dead won't change the happiness they thought was there, but as someone who's still living, telling them lies will haunted you for the rest of your life. like, why tell lies, when in this situation, mizuhara can clearly be already settle with kazuya as the real boyfriend, or if not, just saying that they aren't real couple while also saying that what all kazuya has said before about mizuhara to granma sayuri are all the truth, at least that sincere words are enough to make granma be at peace and happy that there is this person who will protect her grand daughter at all cost and try his best to make her dream come true when she's no longer alive, i think.
tldr; telling all the truth won't hurt grandma sayuri, cause what kazuya has said to her about mizuhara, all are sincere words that will make her rest in peace knowing her grand daughter has this guy who will be there for her when she's no longer in this world.