The text in the Frontier terms applies to flights that stop somewhere but have the same flight number.

So if DFW-ATL-LAS was all one flight number, you would only get credit for DFW-LAS, i.e. 1,055 miles. Sounds like this was not the case for you, so you should definitely get the credit... i.e. 2,478 miles. Unfortunately think you just need to try again and be persistent.