It could be a red herring too yes. We know Eldibus to not only be a near immortal being who has possibly lived for a millennia, but also an emissary on a diplomatic agenda. The fact that Urianger asks him to speak plainly means that not only does Eldibus know how to communicate in the common tongue that everyone uses in Eorzea, but also that Urianger may know how to communicate with Ascians in their otherwise unintelligible speech (possibilities being him having an Echo, learning it from research, being a former Ascian himself, etc). There's also the fact that this might tie far back into 1.0, with Urianger being accused of being one of the mages who brought upon the Sixth Umbral Era:
He's obviously lived for a long time, at least as far back as the early Sixth Astral Era. Him being an Archon is a counterpoint to Ascians -- Archons inevitably are destined to bring about Astral Eras, while Ascians are destined to bring about Umbral Eras. I'm willing to believe they also might discuss this waxing and waning, as it's a direct reference to Hydaelyn eternally opposing Zodiark.
Something like that.