The only part he is wrong about is "starvation mode". It it doesn't exist unless you are literally starving (less than a couple percent body fat). Someone who has 20%+ body fat will not enter starvation mode.
Otherwise he is correct. Diet sodas contain ZERO calories. you CANNOT gain fat drinking diet soda. However, it may cause fat gain indirectly like the OP said such as you think you did good not drinking the regular soda, so you going to reward yourself with 500 calories of ice cream.
There is MUCH MUCH more evidence that diet sodas do not cause weight gain then there is evidence that it does.
Just because it can cause physiological effects and stimulate your appetite does not mean the drink itself causes weight gain. Have a little self control and you can fight the physiological effects.
It is simple does drinking diet soda have physiological effects while you are trying to lose weight? DONT DRINK IT. If it doesn't go ahead. I been drinking a can of diet a&w root beer a day for the past 11 months. In that time I lost 100lbs (or 2lbs/week avg). I use a calorie tracker (myfitnesspal) and you can set it to how much you want to lose per week. Since day 1 I set it to 2lbs/week. I been eating the amount of calories it says I can have per day.. and lo and behold my average has been exactly 2lbs/week.