For future reference, always get an agent to request a refund. While your situation is bulletproof according to the rules, it always is a good cover to have the agent request it because they are UA and by requesting it for you, they've essentially reviewed the circumstances and "approved" it.

I would reply back with the flight information -- yes, they pushed back 30 minutes late, but returned to the gate after 2 hours and then researched a plane swap.