Don't you remember what he said earlier? He came back to the Showa Era.
She's from the future while his gate is from the past or something. He'll have to hand the letter to someone to send like 10+ years later. The person he meets as a young boy there might turn out to be the girl's father or grandfather. LOL. Not sure how that will work out.
He said "resembles" the Showa era.
The manga started in 2024.
He died 20 years ago, so I guess 2004.
We don't know at what age he died. But since he commented how she was just a high school student, he was probably an adult and probably experienced the Showa era.
His base (the place where his gate opened) looked to be in the countryside, as opposed to in the middle of Tokyo.
I'm guessing that's why he felt like it was no different from Showa era.