I dont think anyone is arguing that it was impossible not to gain weight. It broke a lot of good habits people had. Remember it was only 15 days? Some people waited for their routines to open up instead of finding new ones and fell out of habit. Stress also matters. Your body is designed to find and store energy in anticipation of long food threatening problems. It isnt necessarily great at knowing if your source of stress is ACTUALLY food related. So some people stress eat and the weight sticks harder.