The official Hamas number is simply whatever Hamas says it is. There’s no methodology, no oversight, and no reason to assume that missing people are never counted as dead. I don’t think it’s total nonsense because it’s compatible with Israeli estimates of the number of Hamas combatants killed and a 2:1 ratio of civilian to combatant deaths which is in the typical range for urban warfare conducted by an army that is trying to reduce civilian casualties. However, I do think that it is an upper estimate.
With that said, if they know how many people died in these “near-daily massacres” then why wouldn’t they add them to the death toll? If they don’t know, then where are your numbers coming from? It doesn’t make sense.