Sakura said byakugo was from the time of the sage of six paths
She was not talking about the Byakugou though. Sarada was asking about the forehead thing and Sakura talked about the Byakugou and Sarada talked as to why she asked because she wanted to see if it was related to the Karma Seal, which prompt her to bring a NEW jutsu that was from the era of Rikudou Sennin that Tsunade mentioned to her. She was not at all talking about Byakugou.
It's the mark. Even the VIZ translation was talking about the mark. Even called it the Mitotic Regeneration Mark (Creation Rebirth). Byakugou is merely the technique on how it was made.
The mark itself is something different entirely.
My war with people is that they keep thinking that Byakugou is the one that is ancient as it was stated over and over again that CR and Byakugou was made by Tsunade. The Yin Seal is very ancient. Thus I gave examples of what other techniques utilizes the Marking or Seal.
Curse Mark was one of such outside of the Rikudou Sennin bloodline.
To imply that they are talking about Byakugou is to imply that Byakugou is made from Karma.
If it does get retconned that Tsunade just discovered and repurposed an ancient technique, it begs to question on how exactly did she come across it.
Your theory about Mito CAN work as they are all about the fuinjutsu and basically that is how Byakugou works. Sealing ones chakra in a mark/seal. Same goes with Curse Mark.
At this point, Orochimaru may have gotten the information from the same source.
But for now, the one that they are talking about is the Forehead thing itself (Yin Seal) and not Tsunade's technique, Byakugou.
In all honestly, even Senjutsu works the same as Byakugou, but instead of using their own chakra reserve, they just take it from nature, in the case of Hashirama and Toneri or the Yonkai ( like the Legendary Sennins did).