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.
It's simply a measure of time. The fact that it's 365.2425 days, or 31556952 seconds because that's the length of an Earth year is kinda irrelevant.
If the Earth shifted orbit, a light year would still be the same distance.