I didn't see more details in the article, but my guess is they were taking and averaging multiple temperature reads across the body. That is, core temp should only be within a narrow range like you say, but fingertip temp will vary much more widely.

All in all I found this to be a very strange article. If you just look at the data, I think a reasonable conclusion is that modern gear is vastly better at its function than old time Mallory gear. It's much lighter and keeps the wearer much warmer than old gear. But the whole tone of the article is about "myth busting" and how there haven't been really that many improvements in gear. I'm just looking at their charts and data and wondering what they're smoking.