They don't need to necessarily join ow alliance.
Aer Lingus flights, at least on the Atlantic itineraries, would earn tier points (or whatever each program uses) and miles/points in all the JV partners FFP. Also, FFP elites (Iberia, Finnair, BA, American Airlines) would keep their elite benefits when flying Aer Lingus.
I think that's what is going to happen - and the initial plan.
Don't understand the role of Royal Jordanian mess in this though.