Also, to look at the interest of Darkspear Loa in Zandalari from a wider perspective. If they cared much about the Zandalari, why did they wait for Vol'jin's death to make him act in that direction? As per above, Sylvanas isn't really needed under the circumstances of their joining we ended up with. Basic Campfire would get the same results. But, more importantly to the point I'm making, why didn't the act during Cata or MoP when the Zandalari were active?
Think about it this way. Zul turned out to be right about most of his visions and Rastakhan was wrong to dismiss him and leave him to his own devices without his full support. If Darkspear Loa had an interest in Zandalari, they should have sent Vol'jin to corroborate Zul. And try to bridge Rastakhan and Zul, so that Zul doesn't do the stupid thing of turning the world against Troll-kind. Or, when that fails, seek security in the alliance with the Mogu (and once again turn the world against Troll-kind).
Being both a representative of the Loa and the Horde, he could use the backing of the Loa as an argument in favor offerring the Horde as this security, forging this alliance half a decade earlier, saving Zandalari (and other Trolls') lives and making the Horde and the Darkspear stronger. With Darkspear and the Horde also being in a dire situation at the time (Cataclysm, war with Alliance). Especially since, to link to what Aucald spoke about earlier (Loa pushing him to do what he did because of G'huun's threat to them), it is probable that the same Cataclysm contributed to G'huun's seals failing (I mean, it's speculation, but given the effect it had on Zandalar in general, it's probable).
Another thing to consider is that the Loa that led him saw Horde, not the Zandalari, as the home for the Darkspear even before the Darkspear left their island, or made contact with Orcs. Even then they wanted Vol'jin to fight for the proper path of the Horde. They never spoke to him of the Zandalari in the same light. If Vol'jin's deathbed visions had anything to do with Zandalar, it's most likely from the perspective of how that'd strengthen the Darkspear and their Horde family.