Not how I read that dialogue at all. Of course it's hard to say with the translation but since Luna is alive and our actual protagonist is unaware of it, I think her chatting with a dead person makes little sense. It's not like her sister knows about her friends either which she mentions.To be honest, I don't think she is alive, the whole dialogue make them (Tia and her sister Luna) look like a kind of reminder or spirit or flasback or whatever thing it called, but it hint that Tia's death is real, it just whatever soul left of her look Lina and is happy with what she have achive and become a hounorable knight, before truly pass away.
Besides I wouldn't even be surprised if they wanted to put Tia through more sexy situations in any potential follow up stories so I'm not completely convinced she definitely dies in the canon ending but rather I guess it is supposed to be open ended enough to allow wriggle room there.