A successful archival of one of those ISBNs will light up; four of those ISBNs remain dark. Yet they have that content archived. It means that lighting up the entire grid is not necessary to achieve their goal.
Indeed a bigger problem is that it’s much harder to know which areas of the grid are never going to light up because the ISBN has not been used.
Lighting up the entire grid is still the goal, you're describing the problem of ensuring the right set of squares is illuminated for each piece of archived content. One is a problem of archiving the content, the other is a problem of bookkeeping.
Indeed a bigger problem is that it’s much harder to know which areas of the grid are never going to light up because the ISBN has not been used.