The debit of summoning in aos

[Vangrail]

Currently I am at my local game store. We are discussing age of sigmar summoning.
my argument using vampire counts as example

Me = Under zombies it says every death wizard knows summon zombies along every other special u know. So he is allowed to summon zombies even though u start the game with no zombies. Or a summon a zombie dragon ect.

Store manager = u have to have the unit or warscroll in your army to summon it..... personally I think he is wrong.

Thoughts or clarification plz