So...about that Shadowbringers trailer.

[Wyssahtyn]

That cannot be true, or else the White Mages of Amdapor couldn't have created guardian statues in their liking during the 6th Umbral Calamity. I think it was in one of the lore panels that Koji said they were essentially crafted after the liking of some creatures from another world the Amdapori came across. At least that's what I remember. Their statues (the winged lion and Kuribu), however, are clearly based on Sin Eaters, there is no denying that, so there had to be Sin Eaters somewhere back then already.

Tencharacters

As the person who excitedly snapped photos of his Encyclopedia at Fanfest to post on the forum of this detail, I'm aware of what it says—and it doesn't say this exactly. =P The only explanation of why Kuribu looks like it does is one sentence, and the Winged Lion goes without mention: "To better contend with the voidsent, [Kuribu] is said to have been made in the image of transcendent beings from yet another plane." (Encyclopedia Eorzea II, 150) The first lore book's entries for Kuribu and Winged Lion barely mention the reasoning behind why the constructions look as they do other than 'golems made by Amdapor to kill voidsent' save "A winged lion was chosen for the design so as to engender confusion amongst their enemies" (Encyclopedia Eorzea, 294).

There is no mention of Amdapor convening with Sin Eaters, or the transcendent beings. Given what we know now, that seems likely, yes—but there is no hard confirmation that they met or summoned Sin Eaters.