The whole theory behind ballistic tests is that the markings in each case are unique. Like fingerprints and DNA, a match is a match.
Nowhere does the article state that ballistic tests were conducted. Furthermore, IA routinely conducts ballistic tests on the guns of officers involved in shootings.
A barrel is not, as you suggest like DNA, multiple firings will change the markings on the exiting bullets over time. But this is moot at this point. We have no evidence that there have been any ballistic tests.
The officer who said that the bullet "matched Matthew's 'imported' gun" could simply mean that it is of the same caliber. Then again if it was a 9mm that could also mean that the bullet matched one or several of the officer's 'domestic' guns. Although it is also probable that one or several of the officers are shooting 'imported' guns like the Glock 9mm for instance.