So 1-in-30 bullets "matched" the properties of the head-shot fragments?
"This finding means that the bullet fragments from the
assassination that match could have come from three
or more separate bullets. If the assassination fragments
are derived from three or more separate bullets, then a
second assassin is likely."
So--based on the study--there's a 1-in-30 chance that two assassins happened to fire a "matched" bullet into Kennedy's head (the only known source of fragmentation) at the same moment.
More focus on the word "could" and less on "likely".