Dialogue here implies both havent been born yet. Plus the story already established a connection between Luffy and Shirahoshi.
"OUR sovereign will be born soon".
There are definitely 2 lines of thought here. Either the "born" part is referring to THEIR sovereign (poseidon), or it's talking about both. I can see both of these being correct.
But Luffy "met" Shirahoshi already but the Sea Kings talked about how it's too soon yet to move the Noah. Shouldn't it be the apropriate time since the sovereigns have met if Luffy was the one?
I still don't think that's him.
BUt I also don't underestimate Nika wank, so it's always a posssibility.