Ruling Eviolite & Zekrom

[Dark Ho-oH]

I hope this haven't been asked before but I just wanted to be sure about this:

Eviolite says "any damage done to this Pokemon by attacks is reduced by 20"

So, if an Eviolite is attached to my active Zekrom and I use Bolt Strike, will Zekrom do 20 or 40 damage to itself? Because what I understand is that the damage it does to itself comes from an attack? How is it?

 

[King Arceus]

Zekrom will do 20 damage to itself. It is still damage by an attack which is why it is reduced.~KA