Calling yourself out doesn't excuse you.
As to your point, Fortescue wasn't suggesting that a person found guilty should not be capitally punished. In fact, a system of justice that goes out of its way to be sure of guilt, has the right to punish capitally. He was against capricious justice systems.
That said, no point in you arguing we can't be sure. We've (mostly) already agreed on that point and agree that a capital sentence must come with a series of automatic reviews, appeals, and the application of new science as it becomes available.