I get it that you don't get the point why are you bring this up but here's the thing...
If Seething Shore should've happened before both faction are bulking up their forces with allied race, then It doesn't connect pretty well on ATB final scene where Silithus got Stabbed simultaenously popping out Azerites on Silithus and across different islands and shores. They haven't considered approaching 2 naval themed races yet so I think it is logical that they approach the races they have interacted so far first and recruit them asap. They still have anti-Legion mentality incase something goes not as expected. And yep, perhaps preparing for a more sinister fight looming ahead.
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Those allied races that you mentioned are never new races. Apart from Void Elves and LFD, but I'll correlate them later. Highmountain exist way before War of the Ancient, so does the Suramar Elves, although along the 10K period they changed into Nightborne along the way. Dar Iron Drawrves existed way back vanilla and so does Zandalari Trolls.
As for Void Elves and LFD they made a back story from a previously hidden and upfront storyline. back in BC we have High Astromancer Solarian. Way back pre-Legion launch it was hinted that Turalyon was with the Army of the Light so which race that was with him all along in Draenor could be part of such army. The realization of the Draenei Lightforged even became apparent upon Argus patch.
Not everything which is obvious doesn't mean it doesn't exist or never existed at all.