Elemental Archon and Gain Damage As

If you have a split cold and phys spell, gain 400% as extra lightning, then get elemental archon buff, is the gain still calculated from the phys you're not allowed to deal damage as?

If its not, then elemental archon is probably not worth running, even if the majority of your damage comes from an elemental gain.

Does anyone know how poe handles these conversions?