They are talking rubbish, you are right, she doesn't need sweets. My kids never had any when they were little - they got the odd bit of chocolate, they had muffins and pancakes and jam and other sweet food, so I wasn't avoiding all sweet foods. They are now 8 and 9 and although I am more free with what they can have, tbh they are really not that interested. Yes they like chocolate and biscuits, but prefer crisps and when they do choose to have some sweets, they usually don't bother to finish them, and would never really ask for any when we are out. I think they like the thought of them rather than the reality! This year is the first that they have eaten all their easter eggs, but I always have to throw away their halloween hoard as it never gets eaten! They probably eat a lot less sweets than some of their friends, but I don't feel they miss out.
As you say, sweet things are a treat to have when you have eaten the rest of your meal.
It has also had no effect on whether they are fussy eaters or not. My eldest was terrible, youngest was great, now they are both moderately fussy but with different foods.
My eldest also cannot have anything with blackcurrant in as it sends him loopy!! I am sure that has nothing to do with not having sweets. They also have perfect teeth with no decay as well.
You stick to your guns, I am sure that your wee one will make good choices for herself as she gets older. When I let my two choose something to have as a treat when we are in the supermarket, they will either choosecrisps or popcorn or if it is something sweet, it will usually be pancakes or chocolate - and that is with totally free choice.