I think it's fairly simple, Celta's former manager is a Bielsa disciple and former teammate of Poch's. I'm sure he tipped Poch off that this lad is a big talent who fits the system and would be available cheaply.
He's still a teenager and barely features for Celta, so he's not being bought for the first team this season. This is just a matter of snapping up a talented youngster at a good price.
He said the same about GKN & he's struggled to make an impression.