I can't tell when the article was written, but they are using 2013 data, 2016 data is available here:
https://www.atf.gov/resource-center/docs/2016-firearms-commerce-united-states/downloadAnd as of 02/16 lists us with 63,999 NFA registered devices(page 15) - which includes silencers(not available to general public in MN in 2013 and barely in circulation by 02/16).
Note that according to according to the guys on ARF, alot of the destructive devices are flash/bangs registered to Law Enforcement(which explains DC's numbers). Sooo, yeahhh, pretty crappy way to get data. But, as was said before, glad to stay under the radar.