It’s… the topic of this thread. Did you not read the subject of the thread before you started posting in it?
The provided article says the opposite. It shows that ‘function’ is easy, optimized function is difficult.
If it can vary from a number smaller than the effective population size of bacteria, then it is safe to say the low end does not represent a problem.
The authors unfortunately ignored an important aspect of the the D2 domain’s functionality, namely it’s association with two other proteins. The coevolution of these proteins would reduce the difficulty of optimization at any point in the evolution. Regardless, it is a non-issue for any function, as that requires far fewer searches.
Your own article shows that function is high in sequence space when function is defined!