Since you've asked this a number of times, I'll give it a shot. But I'm not guaranteeing that I understand what's going on here.
Overall, what Walcott seems to be doing in this part of his essay is talking about his feelings about his ancestors and what they did to one another. He is talking about his white and his black ancestors both.
I believe that's what he's talking about when he says he gives the thanks that you mention. I think that he can give such thanks because the deeds of his ancestors made him what he is and made his world what it is -- "soldering" the halves of the fruit with its "bitter juice." So he's thankful that his world exists, but he's kind of bitter because he doesn't exactly like the world that was built by slavery and exploitation.
As far as forgiveness or pardon, I think he's saying that it's not his role to judge them and therefore he can't forgive or pardon. I also think that part of it is that pardon and forgiveness are too personal of things for him to give them because he feels no love for them and they are "anonymous and erased."
I hope that makes sense and/or helps...