> It does allow that since letters in a number with radix higher than 10
> are considered digits for the purpose of the rule.

Your comment spurred me to re-read the number syntax section of R7RS. It
actually uses the word "digit" for two different character classes:
<digit> is a decimal digit whereas <digit R> is a digit in radix R (e.g.
radix 16).

So it would be best to rephrase the rule itself (i.e. "conforming
implementations must allow one underscore between any two digits, in any
part of a number") to use a less ambiguous word than "digit".