Based on which claim? That 12 months from now they might accidentally discover a new bug just as serious?
I'm not sure I read anything that makes me confident this class of bugs could never recur. I could be reasonably confident this _exact_ bug in this _exact_ scenario may not happen again, but that only makes me more concerned about variants that may have equal or more serious implications.
So I'm wondering which claim did it for you? I only really saw pen test as a concrete action.