This might not help answer your question...
During Vietnam the tunnel rats were issued suppressed weapons of varying types. It was a challenge to suppress a revolver. One type was a .38 special round with an integral piston that captured the expanded gases. The ATF declared each one of these captured piston rounds to be a suppressor.
I am not a lawyer. It's probably safe to say that if the ATF considers a device a suppressor, so too would NJ. But, like seemingly all firearm laws in NJ, there is plenty of vagueness in the language. "Any weapon for causing the firing of any firearm to be silent." Right.
I don't think the clear line we'd all love to see is here, but I think a key word is "lessen". If you were to do some freebore magic that cut off half a dB at the range, realistically nobody would notice and you'd get away with it... but if you've got negative attention from LE and someone wants you jammed up badly enough, an empirical measurement of reduced sound when compared to-- whatever the investigators choose-- would probably do the trick.
Come to think of it, this sounds a lot like most of the other laws. When was the last time your magazines' capacity was inspected? But if you've already got the contents of your parts box locked in evidence, you can bet someone's going to try to jam 11 rounds into your "10 rounders".