The sword from the ice boss is supposed to let you cast Blizzard;  last time I tried it, it had this same problem.  Not sure if it's been fixed yet as I haven't seen it.

The cuprit to this problem is the spell it to which it refers

            <GameModifier>
                <ModType>Unit</ModType>
                <Attribute>UnlockCombatAbility</Attribute>
                <StrVal>SummonAirElemental_Tatical</StrVal>
                <Provides>Allows the caster to Summon an Air Elemental in tactical combat</Provides>
            </GameModifier>

simple enough to see that it actually does unlock this spell for the play... unfortunately the spell has a pre-req The prereq is the ability to cast summon air elemental. Kind of pointless.... But glancing through the spells a fix to this problem is the following

             <GameModifier>
                 <ModType>Unit</ModType>
                 <Attribute>UnlockCombatAbility</Attribute>
                 <StrVal>SummonAirElemental_Ability</StrVal>
                 <Provides>Allows the caster to Summon an Air Elemental in tactical combat</Provides>
             </GameModifier>

Although this doesn't do the exact same effect as the tatical version of the spell. For example, there is no cost for the spell and there is no knockback or prone when using this summons spell. Currently I'm too lazy to see if I can post a fix for this in the mods folder or not, so you can edit the coreweapons.xml files.