Do AI defenders use the same mechanics as players in Dragonfire?

Yes, exactly the same. The combat engine is identical for players and AI.

AI defenders carry generic role names — Fearsome Warrior, Indomitable Champion, Elusive Hunter, Vigilant Sentinel — but they use real dragon Orders and habits: Bloodscale Terror, Shadowflame, Champion's Brilliance, Dragon's Cunning, Shimmering Mirage. These are the same abilities as the dragons you own.

What that means for you:

• Everything you learn in PvE — the stat triangle, auras, effects, attrition — applies as-is in PvP.
• An AI defender also has a Center spreading an aura, affinities and an Initiative, so the same weaknesses to exploit.
• In PvP, the Preparations screen shows both research sheets, yours and your opponent's — a free scout you don't get against the AI.

So: train your compositions against the AI, they'll hold up against real players.