As egregious as that error is to us, guys, keep in mind that most likely the reporter was told that it was a Thompson and just ran it in the story. "Fact checking" and "research" are generally reserved for the "meat" of the story, not the "color." Time is a very precious commodity in the news world. If the police said it was a Tommy gun, why bother checking that? So he doesn't look stupid to a bunch of guys on a gun internet site? He's got more important facts to check.
Also, isn't that a photo caption? Reporters don't always write their own photo captions. Could have been a page editor or layout person who came up with that gem.
Yes, it's stupid, but we're the 1% of the readers who will notice the error and care enough to think about it for more than a few seconds.