Great information... there was a
thread I started a few months ago with a couple of videos. One showed an offset printing method, but that video has subsequently been taken down. I wouldn't say that the T206s couldn't have been done on offset presses; it's certainly possible it could have with the process being invented a few years before the cards were produced.
According to this British publication from June 1910:
"at the present time seven press manufacturing companies in the United States are building offset presses, in sizes ranging from 14 by 17 to 40 by 62 inches. Other American manufactures are preparing to build, and six plants in Europe are already building offset presses."