Yes, this. And not just flats, obviously, but any mechanical, or even an unexpected relatively mild wipe-out that, in more "normal" temperatures, you would just shake off, rest for a few minutes, and hop back on the bike. The riding itself isn't the problem, if you have the right clothing/gear. It's the "what happens when something goes wrong" factor . . . and speaking as someone who had a near-miss with on-bike hypothermia in *much* milder temperatures, I can attest that it's true what the experts say -- it doesn't take much, or long, to get yourself into potentially serious trouble.