Note that "input" in the description does not mean just any combination of four digits, it means a combination of four digits that is guaranteed to make up a valid time, or it would not be inputted to the function.

No it's probably me who's being difficult, but I have no good idea how to express it in a clearer way. Maybe like this:

Tests pass to your function only inputs, which have a valid answer.

Or: when your function is called, it is guaranteed to be called only with digits which have a valid solution.

Or: Your function will not be called with digits which cannot be used to build a valid 24-h time.

Or: Only inputs which have valid answers are tested.

Does any of these sound better?

These can be any possible combination of four digits which is guaranteed to have a valid answer. it can be 1 1 1 1, it can be 1 2 3 4, it can be 5 6 9 0, but it will never be 5 6 7 8.