To be fair, Wakana just so happened to have read all of Sekai's books / poetry and been extremely into it coincidentally. And assumedly, Nodoka is in the exact same boat(?) Being another fucking nerd that reads books and all. Her reaction to finding out his name is Akira was pretty telling, she probably figured everything out immediately.
I don't really know why his first name would be that important anyway, I feel like I forgot a scene about one of Sekai's poems or something but I'm kinda lost on that honestly. Wakana made the connection from Ami's poetry to Sekai's poetry to everyone have the same last name. "Shrug emoji."