It's something to do with your body getting back to normal after all the hormones etc. Having done Cambridge and had a DS I'd wait! A new baby is stressful enough without managing on 600 calories a day! DS was 6mo when I started.
Please don't subject yourself to 600 calories a day so soon after giving birth. You need all the energy you can get looking after your baby and recovering from giving birth. Personally I'd leave it at least six months - you might find that the baby weight starts to come off naturally anyway.
You've just had a baby (congratulations 😃), that takes a lot out of your body - you grew a whole new human! You need nutritious real food, not Cambridge chemical sugary crap! Honestly, even if you were to go very low calorie. there much better things to "spend" your calories on. Fresh veggies, nuts, seeds, good quality protiens and healthy fats.
Focus on healthy nutrient dense foods for now, and enjoy your new baby