Sorry I’m late to the discussion. I see George’s mention of the confusion relating to our written description in the 2013 article of the orange-billed Eurygnatha from Aruba. We consulted very closely with Ruud van Halewijn on that paper (Ruud also reviewed extensively). I’m taking the liberty of sharing his photos from Aruba of orange-billed Eurygnatha, see e.g.
https://davidshoch.smugmug.com/Seabirds/Thalasseus-terns/i-PkW4thZ/A
I think you’ll agree the bill of this example is *dull* (not bright) orange. (I agree that “dusky” is a poor word choice).
Since then though, I think we under-stated in that paper the potential of Eurygnatha to have bright and extensive orange bills - see e.g. Figures 17, 18 and 21 in this paper in Neotropical Birding
So, about the bird being discussed … honestly I don’t know, but agree with George that orange-billed Eurygnatha should be the null hypothesis. It certainly looks like an Elegant, but I would also expect Elegant to be appreciably bigger than the accompanying Eurygnatha (especially these Caribbean birds).