The calories is from the maltodextrin they use for filler. Only a very small portion is actually sucralose. Pure sucralose has zero calories and a gram of it would be massively too much for regular use hence they add filler, which has the calories.
Of course it says this on the back of the packet... Contents: 1g maltrodextrin
Half-way measures don't get very far. You can only do one thing well at a time. Trying to gain muscle while losing fat will result in failure.
Get to 10% bodyfat first while lifting weights heavy 3x a week with reduced volume. Two weeks maintenance. Bulk up slowly (1 lb weight gain per week max), focus on getting stronger, measure body composition changes often.