Yes that was my guess upon reading the headline. I was surprised they did not even mention this.

It's mentioned, just not in the context of early photography:

  When a camera is produced and we are asked to smile, we
  perform gamely. But should the process take too long, it
  takes only a fraction of a moment for our smiles to turn
  into uncomfortable grimaces. What was voluntary a moment
  ago immediately becomes intolerable. A smile is like a
  blush – it is a response, not an expression per se, and
  so it can neither be easily maintained nor easily
  recorded.