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

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.