#let's go with...

America/Kitty is a ship I have joked about several times before and it's wild how close to canon it is in this novel. Please let them get together in the sequel.

lol no you got it.

But also understand that Sheng Lingyuan is teasing Xuan Ji in the present with that line, because the terms of the bet were never stated in the past.

You can go back to the last portion of Ch.94 to see how this 'bet' is set up. In reality, Sheng Lingyuan only says something vague about wishing that good things could last forever (where it's clear to both him and Danli that he's talking about his feelings for the sword spirit), while he and Danli are playing weiqi. When Danli replies, saying that the only constant is impermanence and, "The Shamans can say ‘remaining constant until death’ because their time of death is not far. But what if they couldn’t die? What if they could live as long as Chiyuan, would they still remain constant until the end of time?", Sheng Lingyuan is discouraged, and gripes with what amounts to "Well how do you know?"

Danli appears to be amused by his answer, seals up their game, and proposes a bet. However, he never states the terms of the bet (although it's implied that the bet is on whether or not Sheng Lingyuan's feelings can remain constant).

(....And, also involves their deeper conflict on how to deal with Chiyuan, Danli wanting to eradicate all kinds that use Chiyuan's energy, and Sheng Lingyuan wanting to shut off the source of energy and allow all kinds to live as one--but either way, it's Danli's unfeeling fatalism versus Sheng Lingyuan's optimistic sentimentality.)

There are two main ways Sheng Lingyuan is able to concretely win this bet:

First, he carries around the remains of the heavenly demon sword all the way to his grave, which causes Xuan Ji to turn back into a sword spirit rather than a pure Vermilion Bird. If Sheng Lingyuan hadn't done this, Xuan Ji's construction of the Nirvana Stone would have been successful (since only pure Vermilion Birds can refine a Nirvana Stone, whereas sword spirits are on a lower tier in the technical rules of Heavenly Law), and he would have never remembered Sheng Lingyuan or tried to save him as time went on.

Second, he risks bringing Xuan Ji to Biquan Mountain rather than going there alone. He does this because he no longer wants to hurt Xuan Ji by making arbitrary decisions on his own. He decides to put his relationship on equal footing to the bigger picture. If Sheng Lingyuan hadn't done this, Xuan Ji's body would have been destroyed when Chiyuan reignited, and Xuan Ji would have returned to the state of a wandering ghost and followed after Sheng Lingyuan. Now, Wanfei wouldn't have won, because Danli was always there waiting behind her, but Danli would likely have had to use both Wanfei and Sheng Lingyuan as sacrifice to return Xuan Ji's soul to his original body.

Thus, if Sheng Lingyuan hadn't done these two things (both of which relied on his feelings for Xuan Ji remaining constant), his soul would have scattered (but no matter what, Danli would have ensured that Xuan Ji returned to the state of a pure Vermilion Bird).