We don’t know that. We don’t know what “point five last lightspeed” actually means. It could mean 0.5, or 50% faster than the speed of light. Or it could mean something else entirely. Maybe it’s some kind of exponential factor?
Plus they aren’t traveling at lightpseed or what have you in normal space. They do it in hyperspace. Which is never explained but could very well be some sort of quantum space-bending wormhole or some such. So traveling at anything resembling the speed of light (or more) through something like that could result in insanely high relative speeds from one system to another.
Plus it’s space fantasy that has never concerned itself with real world physics and has never given enough info to really be able to relate it to or calculate it with real science.