I find the 1200kcal => no weight loss claim dubious. However, there's more than two options, because "I eat X" doesn't necessarily mean "at BMR which would not

I find the 1200kcal => no weight loss claim dubious. However, there's more than two options, because "I eat X" doesn't necessarily mean "at BMR which would not vary with diet, I metabolically convert X at the caloric density listed on the label for the macronutrients involved." A cup of black coffee has 5 kcal. Surely you would agree that consuming 1200 kcal/day of just coffee for 6 months (if you somehow survived) would lead to a different weight delta than consuming 1200 kcal/day of, say, non-dairy cream substitute?