The GPS antenna on the LTE tower should only be picking up signals from the sky. Transmitters on trucks would need to reflect off of aircraft in order to cause trouble, which would greatly weaken the signal.
Yes the antenna pattern will attenuate the signal coming from the ground somewhat, but it also doesn't suffer from 182 dB of freespace attenuation like the real signal. It doesn't have to bounce off of an airplane either, the Earth has a layer of atmosphere around it that gets ionized by the sun and is also full of water vapor. We also don't have the ability to build perfect antennas, especially since it has to cover the entire sky (GPS antennas do not physically track the individual satellites).