You're considering only the speed of light. You need to factor in the chosen unit of time as well (the year). If the thargoids used the same unit of measure it would likely result in their light year being different to ours. The speed of light would remain the same, but their light year would be different due to their home planet's different length of year.Umm... no, a "light year" is how far light travels in a year. It has nothing to do with the orbit of the Sun or any terrestrial things.
It's a universal constant that exists throughout the universe - roughly 186,000 miles per second.