You aren't wrong, but that is because they don't know how to science. When they say "measure" they mean "laser" and that isn't the only speed of light measure. How fast do EM waves travel? "Data" can be measured several ways, and in unidirectional travel. Conditions: set a clock on a spacecraft/rover send spacecraft/rover to distant place calculate time shift due to travel - it will be either known or constant now there are 2 clocks a distance apart. program the spacecraft/rover to receive a time signal and return the clock value when received and the difference in the times. program the spacecraft/rover to send a time signal at a scheduled time. this could be done right now with the rover(s) on mars. test: send the current earth time to the spacecraft/rover on the hour, like 12:00 data travels at speed of light to spacecraft/rover. time received is marked and then subtracted. this value is sent back to earth. at a designated time the spacecraft sends local time back to earth, this time is received and the difference calculated. Mars would be a good test platform because a) it is far enough away that relativity time shifts would be essentially insignificant compared to the speed of light travel time. b) the distance varies in the known orbit, giving data from relatively close to very far away, c) it allows for omnidirectional testing of the speed of light over the course of a couple years, providing a full picture of the speed of light in all planar directions.