Maybe its in reference to the flood on the shard the WoD came from? Minfilia wasn't able to prevent the flood and this other voice is someone from there trying to get more help?

A few other points for consideration. There's an underlying theme of going to war with Garlemald and nobody having enough manpower to effectively win that fight. The voice asking for us to open a gateway so they can come could be leading to a situation where we'll have refugees from another shard that join us in the fight against Garlemald.

You mention only Hydaelyn has pulled us into the sea so far. Minfilia is effectively an avatar of Hydaelyn now, its quite possible she could pull on Hydaelyn's power to project Arberts voice to us in warning.

If it is Arbert its possible they've gone through similar events on their world and its not that they can see the future but they have previous experience that lets them know the path we're on won't work. Or possibly Minfilia is able to see the future and influence Arbert?