Where did you ‘hear’ that?
Im growing it no winter damage last year. Isons says Z6.
An ‘expert’ says Z6 Can I grow marionberries? - Ask Extension
Several nurseries say Z4 which is most likely wrong… a few nurseries say Z7 which is probably a safe bet for them… but i think it will likely be ok in Z6 and above barring a polar vortex or something.
Barring all of that… Obsidian should be an improvement over Marionberry if we can figure out why it grows as if its diseased for all of us… if it clears up it should be double the yield of Marion… and similar in cold hardiness.