Do you mean that specific folks told you they would not release their
data without such a clause?

I had always imagined that the clause was effectively toothless, I
can't imagine that you could prevent people getting the data even once
you have demonstrated that they have identified it. In any case, by
that stage it is too late. Is the idea that the people using the data
will be frightened to de-identify it because of the license? Who
would prosecute them?