My guess is not only Spaniards will apply, but also Latin Americans with European passports (like me). Being the main airline connecting Europe and LatAm maybe they’re interested in taking in a couple people who aren’t Spanish? Of course the total opposite could be true, and they could be looking just to hire nationals. I’ve never figured out if Iberia is open to hiring non-Spanish people, my guess is if you meet the requirements you’re fair game. I know other IAG airlines like BA and Aer Lingus have taken in people from all over Europe, hopefully Iberia will follow suit.