Matches that were called something like one in 8 billion were sometimes more like one in 2 million. In a city the size of New York, that's far less certain of match than they stated.
The expert points out that it is 1 in a million. The jury will think this means that there is a 99.9999% chance this guy is guilty.
But how it works (assuming the DNA is the only evidence to connect this guy to the crime) is that in a city of 100 million, there are 100 possible people who would match, so the actual chance this one guy who was found by DNA match is actually the guilty one is only 1%.
People are willing to convict if given 99.9999% certainty. But who would be willing to convict with only 1% certainty?