Chthonic Steel says of armours made from it:
Chthonic armor and shields negate magic missile spells that target the wearer in a manner similar to the shield spell.
How would this fare against Mythic Magic Missile:
The damage dealt by each missile increases to 2d4+1. The missiles bypass the shield spell and similar effects that block the non-mythic version of this spell.
Most of my hesitation in stating "MMM bypasses the protection afforded by the chthonic steel" stems from the vague nature of the text in the steel description.
Is there a section of text that better clarifies the intent of this quality?