Of course a tintype is a different technology from fast film or plates, or whatever this photographer was using.
However, you're incorrect about the lack of usage in the timeframe specified: "Tintypes enjoyed their widest use during the 1860s and 1870s, but lesser use of the medium persisted into the early 20th century" https://en.wikipedia.org/wiki/Tintype
Besides, the GP comment referred to the era, to which I was responding.
Your explanation is not without merit, however: "One common explanation for the lack of smiles in old photos is that long exposure times — the time a camera needs to take a picture — made it important for the subject of a picture to stay as still as possible. That way, the picture wouldn't look blurry." — https://www.vox.com/2015/4/8/8365997/smile-old-photographs