Disregard me: This is an inaccurate statement, not an understatement. I still maintain the understatement of the century probably happened sometime around WWII.
Better candidate:
This morning I had another talk with the German Chancellor, Herr Hitler, and here is the paper which bears his name upon it as well as mine.... We regard the agreement signed last night and the Anglo-German Naval Agreement, as symbolic of the desire of our two peoples never to go to war with one another again. - Neville Chamberlain
Your overall point is well taken, but you're interpreting "century" to mean "last hundred years". The way I'd interpret the word, this is a candidate for understatement of the last century. And the winner was probably somewhere near the Trinity test...
From the final chapter of his new book Cowards, titled "Adapt or Die: The Coming Intelligence Explosion."
The citations for the chapter include: