What I meant was how Maya learnt about Sekai instead of knowing her personally, if this is what confused you. Maya may have learnt about Sekai on a rather detailed level by being the witness to him talking to hallucinations, as I bet Akira did not have what it takes to talk about any of it, not now, not then.
But does Maya know Sekai personally?
Maya for sure acknowledges her as more than Ami's mom, but I don't see a chronologically reasonable way for them to meet (assuming both of them are and have always been plain human beings).
IIRC the only piece of material that has Sekai "directly" interacting with Maya is in
Something Everyone Knows and Ignores, where Sekai can be seen being very toxic to toddler Maya. But I really don't know how many things you can extract from happy events and interpret them literally, so I'll put a pin in it.