What should we make of the outspoken defense offered by William A. Ackman of his hedge fund’s awful performance in the last two years? Has he, as he claimed at last week’s DealBook conference, learned the hard lesson of his disastrous 2015 investment in Valeant Pharmaceuticals International, which has cost his investors billions of dollars? Pershing Square Capital Management, the activist hedge fund Mr. Ackman has run for more than 12 years, has had a difficult two years. Its publicly traded entity lost 20.7 percent so far this year, after losing 20.5 percent in 2015, while its private portfolios are...