Caption: The vacuum light speed c = 2.99792458*10**8 m/s ≅ 3*10**8 m/s = 3*10**5 km/s ≅ 1 ft/ns and relative velocity.
Features:
How can this be?
The slightly longer answer is that length and time flow rate depend on relative velocity according to special relativity in just such a way that both Aliens measure the vacuum light speed to be c.
There is a sort of "cancelation of paradoxes".
By the by, mass also depends on relative velocity---but that's another story
The mathematics of these dependencies is simple. Wrapping your head around them is hard.
All of the special relativity depends on the invariance principle and if it were wrong, all of special relativity would be wrong somehow---but it is all right to within experimental uncertainty.
So we have overwhelming confidence in the invariance of the vacuum light speed.
The weird effects of special relativity usually scale as v/c or (v/c)**2, where v is relative velocity. In everyday life and in many other cases, v/c << 1 and the weird effects are unnoticeably small.
But they are real enough and can be measured with accurate/precise equipment. For example, Global Positioning System (GPS) would NOT work well if special relativity (and general relativity too) were NOT accounted for. In fact, GPS confirms special relativity and general relativity every second of the day.