Expansion site: https://www.guildwars2.com/en/secrets-of-the-obscure/
Trailer: Click Here
Announce Blog: Click Here
Combat Updates: Click Here
Story, Mastery and more updates: Click Here
ThatShaman Countdown: Click Here
After the Dragon Cycle - Guild Wars 2 News: Click Here
First Contact: The Kryptis and Rift Hunting: Click Here
They specifically say that you can equip those weapons as as soon as you unlock Weaponmaster Training in SotO (whatever unlocking means) without training a (any?) Elite Specialization.
I don’t know whether this means that you do not even need to own the corresponding Expansion. I think it’s really weird.
They explicitly say that you don’t need other expansions, if you only want to play with those weapons. But it will NOT unlock the elite specialization. So: you want a long bow core guardian with only core game and new expansion? You can.