As far as I understand things that's a deeply buried memory of Sion's, which would've been from when he was alive. Sion doesn't have the best track record for remembering his time as a mindless abomination, and is completely unaware of his current state of being.
Also given how the lore reads it pretty much seems to come immediately after his re-ressurection at the hands of Swain, Vladimir and LeBlanc. I don't think it's physically possible for the General to have actually met Sion during this time, since he went from his coffin to the fields of war and then back to mindlessness all within the span of a day, with little time between said events.