Some people are latching on to the time travel theory, but are suggesting that Bort traveled to the past and witnessed the original Ten-Tails instead.
One theorist I watch has even cited a single panel from the fight against Kaguya, where a shadowy figure who is unaccounted for is seen in the background of Team 7+Obito's fight against Kaguya...
...Which, if that
is a time-traveling Bort, would suggest that this was planned for nearly a decade.
Now, the anime version of this scene is framed so as to imply that was Sakura, but the anime also changed the typography of Kaguya's gravity dimension and the mysterious figure appears to be wearing a cloak or cape in the manga which Sasukra was not.