It all depends on how trusted Takemura really was. he seemed to be truly and unpleasantly surprised that Saburo had his engram. Of course, it is possible he just played dumb. he also dies after the parade if V doesn't help him.Its possible, but unlikely he knows little about her by 2077. She is a VIP, basically the 4th most important person in the company at worst. And by 2077 and active participant(board of directors) . Saburo would have files, His personal body guard would likely have a folder on her at the least. Her life being kept secret was a detail that was true in her youth, by the time she was 17 Arasaka was aware of her existence, which is like 50 years ago by 2077
If he was just like, I don't know her personally it would be one thing, but he was talking very generally about hanako and yorinobu, Then he is like, I don't know anything about her. He at least knows she is American born, raised mostly in America, and recently joined the board of directors. He definitely knows she had her own private investigation company.
Its like a star wars fan being like, Luke is a powerful Jedi, Leia lead the rebellion, and I don't know much about Han solo. Even if you know a lot less, that description makes it seem like you got no info, and thats very unlikely.