In late March, popular consensus was that Japan was managing its coronavirus epidemic well, and was unlikely to be seriously affected. Patrick McKenzie, a software entrepreneur in Japan, thought that that was wrong, and on March 23rd through 25th he and three other locals produced a white paper describing their reasoning, which they shared with Japanese news organizations.
They were right; Japan is presently in a state of emergency. According to Patrick, their work helped accelerate the Japanese response.
(You may recall reading on Marginal Revolution that "The coronavirus situation in Japan is probably much worse than you think"; this is the group Tyler Cowen was talking to).
This essay describes how he reached his conclusion and how he acted on it. It's rather long and elliptically written, taking great care not to attribute blame to any of the institutions involved. Read it anyway: it's an account of concrete, successful action against Covid-19, and seems to contain a few LW easter eggs for alert readers.
Note that you have rewritten and cherry-picked his predictions.
His precise predictions were all wrong. Maybe Japan was undercounting cases by 5x, but so was everyone else. Cases were rising at 8% per day and deaths were rising at 8% per day. Cases and deaths have both continued to rise at 8% day. For factual purposes, the best prediction was to trust the data and simply extrapolate.
The consensus that Japan was OK was wrong, but it was directly contradicted by the official data. Exponential growth is bad. 8% per day is unacceptable. But would it be possible to simply point that out? I don't know. Maybe the only way to get attention was to claim that the data was wrong.
Here is another story of people refusing to acknowledge exponentials.