If the chat was in his archived chats, it means there has been an active WhatsApp chat at some point between DP and AP.
A contact would not appear in the active list of current chats or in the archived list of chats if no chat had ever taken place.
DP has cleared the contents of the chat and then archived the chat.
Unfortunately, there is no way to know exactly when the chat took place. All we can know for sure is that he blocked AP in July. Therefore, the chat must have taken place before July.
It is possible that the chat was in January when you found out about the affair and there has been no conversation since.. however, I would question.. why did he not block her in January then??
It is also possible that they maintained contact up until July when he blocked her. Sorry OP, but this seems like the most likely conclusion.
Either way, you deserve better! DP has clearly broken your trust which is causing you to snoop and check his phone. That is no way to live. Suspicious and anxious all the time. You can't be happy. If the trust is gone, why are you still with him? Don't waste your life on a cheat, go and find happiness with someone who deserves you! 