The DP was not the petri dish you think it was.
The people were not all huddled in the Grand Ballroom hacking on each other. They were in separate cabins with separate facilities in different zones of the ship and practiced isolation, social distancing and prophylactics such as wearing masks. Some of the sick were removed from the ship, another form of isolation.
If we could get a breakdown of where in the ship the infected were, I would be willing to bet that some areas were heavily affected while others were not affected at all.
The ship ventilation system was the most likely vector by which Wuhan Virus reached the passengers. The crew infected each other.
This was certainly not as cramped. crowded and dangerous as the quarters on WWI troopships carrying American troops to Europe, or trooptrains rolling across America. Yet somehow the same proportion of the crew+passengers on the Diamond Princess got Wuhan Virus (22%) as was common for American soldier passengers on the 1918 American troopships and troop trains (20-25%).
The Diamond Princess experience is most useful in determining the proportion of victims showing symptoms to victims with no symptoms (3/7 vs. 4/7).
They also threw the towel in and eventually let the uninfected out before they all became infected on the DP.