We don't know that, and we can't prove it. That's a dangerous rhetorical approach to take.
There are many factors that could potentially cause the drops in crime we've seen. Correlation is not the same thing as causation.
It seems the Anti-gun people are big on rhetorical approaches. It took me a long time to understand the vary last sentence of this quoted statement.
I feel if we keep dealing in facts and try to get the anti-gun people to put away feelings, then we make a little bit of progress.
If we continue to cite facts, and this report will help, we can help influence peoples opinions. Most people seem to be low-information voters and will listen to a 30 second commercial, but will not read a report on something they claim is of big interest to them.