"Everything in moderation" implies that you do not have to give up your favorite foods but you should be careful about how much you eat. The evidence comes from the energy-balance equation: if you take in fewer calories than your body needs then the body dips into its own energy stores (often fat stores) and over time, the scale shows a sustainable weight loss. Therefore, you can eat any food as long as you don't eat more calories than you burn. The evidence is that weight loss follows a reduction in calories, i.e. the tip's evidence is based on physiological rather than behavioral evidence.
However, if by trying to be moderate, you commit to serving yourself just one handful of potato chips AND you can't stop at one handful then you have exposed yourself to more calories than you needed and your weight loss goal just slipped further away. The "Everything in moderation" mantra just backfired: Your behavioral tendencies ruled the day. You may have been better off not trying to be moderate with the potato chips.
Gretchen Rubin, at the Happiness Project blog, talks about the usefulness in knowing whether you are a "moderator" or an "abstainer". According to Gretchen, some people just do better if they go cold turkey when trying to make a key behavior change. For example, an "abstainer" might be more successful by not having any potato chips at all. A "moderator" might feel deprived going without potato chips so just having a 1-handful portion might be a more successful strategy. Knowing whether you are a "moderator" or an "abstainer" might just help you figure out why your calorie reduction strategies haven't worked in the past and develop the right strategy for the future.
Or even better, as an "abstainer" you can smugly feel confident and guilt-free when your "moderator" friend admonishes you with "Everything in moderation".