I have had periods where my diet totally relapsed but my exercise never did. That is why I think (most) people need mental, exercise, and diet based strategies to lose weight. If you only use diet and that gets sketchy for a while (life stress, whatever the reason...) then your chance of relapse and weight gain are much higher. When just one piece of your strategy falls away if you still have a few other legs holding it all up it will probably stay standing.

Plus I have met distance runners with ridiculous diets. Some of them run 50 miles a week just so they can eat a pint of ice cream every week and pizza multiple times per week. It happens, but those are always at the extremes. A balanced approach is needed for most mortals.

People will justify their beliefs in a whole bunch of different ways. Until someone has actually lived it and dropped 40+ lbs and kept it off for years I immediately down grade whatever advice they are giving as they are simply less credible (unless they are well versed on the actual science and literature of the topic, in which case they likely never gained excess weight to begin with or fall into that category of someone who has lost weight and kept it off).

The number of people I have met or know personally who diet it all off with no exercise and are convinced how right they are "I lost 100 lbs!" only to balloon back to their start weight or close to it is ... a lot.