Note that the people wo did this, used the 18 bit machines before.
As steve johnson pointed out, just about everything around then was octal. I think IBM brought widespread use of hex with the 360 ca. 1964, but to most other vendors octal was a way of thinking as your interesting quote points out.
As for character formats, on e.g. the pdp-10 you had lots of choice, including 6 6-bit chars or 7 5-bit chars with one bit left over as common ... I also recall people used to complain about the inefficiencies inherent in 8-bit character formats ...
I for one was still pretty glad to see octal mostly go away.
ron