The effect is 5% spell damage absorbtion for max of 300hp (so, you get hit for 3k damage from a spell, you only take 2700hp damage -- 3 ice commets, etc...).
If it's not a proc then how does that work?
A) 1 shot wonder where 300hp worth of absorbtion and it goes bye, bye?
B) 5% continuous and to hell with the 300hp limit?
C) 5% when you equip then after 300hp damage you have to unequip and reequip to get it to work again?
As a proc it matches the proc listing and would require no changes nor fancy code to get it working. As a 'worn effect' it doesn't fully match because 'something' has to give -- be it hp limit (most probable if it is worn), an auto-disolving augment (doubt it or folks will get a new definition for 'wasted points'), or equip/unequip/reequip (doubt it but possible).
Not saying you are mistaken about the proc being bogus but it does change the value of this item if it isn't a proc...
Actually, most pre LDoN weapons have type 4 slots. Also, just because its a weapon augment, does not necessarily guarantee its a proc. You may be right on that, but it sure would be sweet to toss on my clicky stick, if its a permanent effect.