Kinda weird how the Isekai girl said she is his GF when he died in the other world before confessing. It definitely different from the one shot. I prefer the way the one shot do it but we'll let see
I am pretty sure that the alt universe short hair version one is lying about being his GF, and that she is just saying that as wishful thinking after losing her world's MC.
That's what I thinking too. We can see her blushing on page 54 when the original talks about the MC confessing so I think she didn't know if he was in love with her or not.