I would say the author is also confusing the words "digit" and "radix".

   From The Free On-line Dictionary of Computing (30 December 2018) [foldoc]:

   radix

       <mathematics> The ratio, R, between the weights of adjacent
       digits in {positional representation} of numbers.  The
       right-most digit has weight one, the digit to its left has
       weight R, the next R^2, R^3, etc.  The radix also determines
       the set of digits which is zero to R-1.  E.g. decimal (radix
       ten) uses 0-9 and each digit is worth ten times as much as you
       move left along the number.

       (2006-11-10)

Admittedly, "digit distribution sort" might be a more descriptive name for the algorithm.