Please, just start a game (with all DLC enabled and no mods) as e.g. Rurikid in Holmgardr in 867 (northern Russia). Declare war on someone, then call your vassals to war. He's at minimum tribal organization at the start and can call his vassals to war without problems.
Play a bit further and increase tribal organization. Again declare war, call tribal vassals - no problem.
People are disagreeing with you because your statements in this particular case are simply not correct. I've just spent many hours in the past few days playing as Holmgardr (turns into Novgorod upon expansion), working my way up slowly to the Empire of Rus and preparing to go feudal, and no matter what my tribal organization was, I could always call my tribal vassals into my wars. From min to max, makes no difference. It only takes a few minutes to try it out and you'll see that I'm right.