My doubt is whether a digit can be inserted where there could be a word, but not one that produces a solution.

Well. I think it is perfectly clear. You cannot use a digit at any point where a word is available. (*)

(*) I was about to add 'regardless of whether a solution can be achieved', but it is unnecessary. If there is a word available, you cannot ignore that word and use a digit. For any reason, including achieving a solution that would otherwise be complient.

For the record, without that rule, there would be many more solutions that are presented in the official results.