Quoting vieuxchat, reply 18Devs said in another thread that it would be simultaneous turns.
As I understand the Impulse hosted games it will be sort of PBEM then, you just don't send your turn as email attachment but via Impulse / from within the game.
Where is that thread? I couldn't find it :/
End of alibenbaba's quote
If it will allow multiple players to take their turns at a schedule to their liking (24h,48h etc) then this would indeed be equivalent to PBEM.
It will have the drawback of the making the whole player community dependenant on this server. Server breaks => No MP for whole comunity = 
PBEM is more flexible but centralized server that emulates PBEM MPing would work as well and indeed be awesome 