I’d agree with you if the call was Spell Strike Drain/Purify. But it is a weapon strike which are not even supposed to be magic. It is moderately ambiguous like calling 1.3 alchemical cause damage stuff necromancy.
There is nothing ambiguous about 1.3 Alchemical Cause Damage. It is explicitly not a necromantic effect. That is part of the reason that it is called "Cause <X> Damage" as opposed to "Cause <X> Wounds." The change in name was supposed to help prevent confusion.
An effect does not have to be magical in order to be necromantic. That has been true since back in the NERO days. Carrier attacks, for example, aren't magical. However, if a creature is swinging "2 Disease" then the creature is channeling necromancy. Heck, theoretically, if a creature had a packet attack that was "Wither Limb Poison" that would also be necromantic. I've never seen that, but the rules allow it and necromancy is identifiable by the key word "Wither Limb" in the effect.