I presume from successive Doppler they have been able to deduce a reasonable accurate speed range hence the track band was determined and the cessation of transmissions determined the track box length.
I haven't seen any claims along that line; I think they are delighted just to be able to tell if it was moving towards the satellite or away from it. That's how they knew it went south instead of north.
The speed is calculated from a 777s' performance envelope (they knew how much fuel it had, and when it presumably ran out.)