Even from a lore perspective, a PLD aura can be explained. Actually, it would sort of make sense because:
PLD Job Quest Spoiler
Heavensward Job Quest for PLD is specifically about returning the aura to Oathkeeper, and Syl-what's-his-name uses Constraint & the Warrior of Light to further this goal. In the end, the Sword will only glow for the Warrior of Light, who passes it along to what's-his-name so he can strive to make it glow for himself. The glow (ie the Oathkeeper) is actually the Soulstone and the Paladin whom carries it, not the sword itself. This is why the Sword glows for the Warrior of Light and not the others. The WoL themself is an Oathkeeper (ie a Paladin better than the rest, free or Sultansworn).
Actually then considering this: a real lore-friendly approach would have been Royal Authority to be the lvl58 skill, and Divine Veil being the lvl60 (with an Aura FX attached). I say this because the 'magical' abilities of Paladin - Clemency & Divine Veil - seem to be drawn from the Soulstone, and are abilities only the WoL PLD can perform (Constraint and Syl-what's-his-name use Spirit's Without and some other ability similar to Clemency, but with a different name)