I've witnessed the result of someone not being visible to a taxying aircraft being hit, "almost certainly directly as a result of them not wearing the hi visibility clothing" (the coroners words, <himself a very experienced G/A pilot>, not mine).
Presumably not therefore
a) the fault of the pilot for driving into him
or
b) his fault for walking into the path of something noisy and dangerous, which, being airside, he might have anticipated could be there.
The fact that he wasn't wearing the jacket does not IMHO make him at fault for being hit nor absolve the person who ran into him from blame.
(written from the perspective of knowing no more about this particular case than has been stated in this thread)
Mike