Sothis being an adult is a brainlet filter to trick people into thinking it's a sequel. If you have a peanut sized brain and only watched Three Houses on youtube you would think her being older means time has passed.

Now, if you actually played Three Houses, you would know she was old in the past, was resurrected younger, and then got absorbed into Byleth. That means you know that Sothis was old in the past and young in the future, so her being old actually implies this is set in the past.

For people who have actually played Three Houses, Sothis being older doesn't mean anything, because Sothis' age already doesn't track with normal chronological time. She comes back younger.