In any case, he didn't see it and got clipped.
Apollo's depression could have been from lots of things, but this was his "boiling point". He said he would be backing up Starbuck, but he blew it and wasn't there. He failed. There was nothing he could do about it but in his mind, whether or not it would have happened, he wasn't there. Another thing was he was ready to die out there (well he did...). Whether he let go from lack of energy or because he wanted to, he let go.