Technically, GPS uses both pseudorange and doppler data as an input. These are usually combined with the previously measured position and speed using a Kalman filter. Speed is generally more accurate than position (to the extent that it makes any sense to compare them) because pseudorange errors tend to change comparatively slowly. Of course how the receiver outputs the speed is up to it, but since it falls out of the position calculation for free it's unlikely many will be storing straight delta position/delta time.
Having said that, it's not especially strong evidence since it's pretty trivial to fake. However, in this case the title is correct: it contributed to him getting off because it gave him the confidence to challenge the ticket.
Sure it's easy to fake, but you are under oath. You can stand up there all day and say that, to the best of your knowledge you were not speeding and not perjure yourself. If you get up there with forged documents and get caught, that's something else entirely. So, they are not iron-clad evidence, but they should be pretty good.
Having said that, it's not especially strong evidence since it's pretty trivial to fake. However, in this case the title is correct: it contributed to him getting off because it gave him the confidence to challenge the ticket.