[Zappin!]

No, unfortunately in this circumstance, the FAA denied boarding compensation rules would not apply. These rules only apply when you are involuntarily denied boarding due to an over sale of a flight.

It's true that when you are not at the gate in time for the original scheduled departure time, even if the flight is delayed, your seats are subject to cancellation. It seems there were many weather delays in Atlanta that evening.

With weather delays, agents are literally standing by the phone ready for a call that says "Hurry up and board now because we have a takeoff time slot in 40 minutes!!". On a weather day, there are probably several standby passengers who's flights have cancelled and are hoping for seats to open up. Agents want to have those flights ready to board at any moment in case they get that phone call so seats of those not at the gate are released and given to standby passengers in case boarding needs to start immediately.

You technically weren't denied boarding. You missed your flight due to a weather delay. In this case, they rebook you without charging fees, but obviously lodging is not provided given it is a weather delay.