I do not agree with him. We can't tell from the Greek to whom the words were being "directed", but we can tell that nobody, neither Jesus nor Jah was being addressed.
He is correct in saying that when one was being addressed as "Lord" the Greek word "kurie" was used. That's the vocative case.
English example: "Joe, bring me the hammer." "Joe" is vocative. Joe is being addressed as "Joe". It's different from "Joe brought me the hammer." In this case, "Joe" is nominative case. It's the subject of the sentence. Joe is not being addressed as "Joe".
Greek example: "My lord [Kurie], you are the one that knows." The elder is being addressed as "lord", so "lord" is in vocative case.
At Joh 20:28, the vocative case "kurie" is not used, so, in my opinion, no one is being addressed. Jesus is not being addressed, and neither is Jah. I used to think that maybe when John realized what was going on, he looked up to heaven and referred to Jah as bing "my lord", but I don't think the Greek allows for that. As I said, since "lord' is not in the vocative case, I don't think anybody is being addressed.
What case of the word "lord" was used at Joh 20:28? "kurios". Nominative case, the cased used for subjects and predicte nouns. Examples: "Joe brought me the hammer" (nominative case - "Joe" is subject of sentence). "My cousin is Joe" (also nominative case, but here "Joe" is not the subject, but is a predicate noun).
So, main point: At Joh 20:28, evidently nobody is being addressed since "kurios" is nominative case, not vocative. It seems that the word is serving as a subject or predicate noun. I tend to think it was serving as a predicte noun and that John was actually saying "you are my lord [kurios]" with the words "you are" not stated, but understood.
It would be like this. Suppose I live in medieval times and there is some dispute as to who is the king in the land in which I live. I witness a man perform an act that proves he's the king. I look at him in awe and simply say "my king". I'm not addressing him as "my king"; I'm meaning something like "you are my king".
So I think John was saying, in effect, "you are my lord". Now, we are still left with the issue of at whom he was looking or to whom he was directing the words. My first guess would be Jesus. But I believe it is possible that he could have, at the moment of his realization, looked up and said to Jah "my Lord and my God", meaning "Jah, you are truly my Lord and my God".
However, the latter doesn't seem as logical because as far I know John wasn't doubting whether Jah was Lord and God; it was Jesus he was doubting. If John had questioned the existence of Jah and had seen Jesus perform a miracle, then I feel that the latter could have made sense. Seeing Jesus perform a miracle could have increased his belief in Jah. But again, I'm not aware of his doubting Jah.
The JW's point about Peter is not even relevant as far as I can tell. Peter was being addressed as "Satan". The Greek word was "Satana" (vocative case). But nobody believes Jesus was saying that Peter was literally Satan. What's the guy's point? I'm not seeing it. Peter was addressed as "Satan" but I agree that he isn't Satan, so what? What's his point? Nobody was being addressed at Joh 20:28. The Greek word under consideration is nominative, not vocative. I see no relevance.