Saving the world is a Good act when it is difficult, dangerous, and you have a perfectly valid Neutral option to escape the world available to you.

This is the option that Hilgya lined up for herself. She gave a little push to the "saving the world team" but stopped before it became too strenuous or a real risk to her life. If it turns out not to be enough, then she can simply escape to another Plane.

Serini and Belkar can't do that under their own steam. Belkar theoretically could abandon the Order and go off looking for a high level caster to warn and become escape buddies with (maybe even convince Hilgya), but he's not inclined to do so because, like Xykon, the world contains many of his favorite things. Plus, he prefers a dangerous lifestyle by default and would confront dangers even without the motivation of saving the world. Nevertheless, if push came to shove, and he had a choice between near-certain obliteration and a small chance of helping, versus jumping clear of the Snarl implosion, then if he were to stay and help, that would probably earn him some Good points.

This Neutral option is much more viable for Serini. She's had more time to prepare, has known more high level people, is capable of using more magic items. If it weren't for the line "[Existing] was nice" then we might even expect her to have an escape plan in her back pocket, regardless of her alignment. But I don't think she would say that if it was her plan to pull a Hilgya and abandon ship. Whatever her plan, she intends to see it through, and that makes me think that (pending new revelations that her goals are not what we have been led to believe) she is Good.

Not solely because of points earned by committing her life to saving the world, but because she is the kind of person (currently, even after her embittering years) to do it.