What that site actually says is
You don't have to avoid sugar completely. As long as your everyday diet is healthy and generally low in sugar, some sweet food (in small portions) will do no harm, particularly if eaten as part of a meal.
Sugar, in the form of table sugar contains only calories and no other nutrients so it isn't very good for anyone to eat in large quantities or regularly. However, fruit, vegetables and nuts, and milk all contain sugar, albeit acompanied by more useful nutrients. Unless you avoid carbohydrates completely ,you will consume some sugar when you eat these things.
Since the body doesn't discriminate between the source of the sucrose ,eating small amounts of food containing added sugar, occasionally and as part of a meal is possible, The amounts that won't cause problems is very much a 'your mileage may vary'. Personally I feel it may depend very much on your activity levels.
Sid's truffle was an occasional treat. I think that you will also find that there are many people who like to eat a square or so of dark choc after/as part of dinner.
If you want to make a dessert from time to time, it is possible to find desserts that include very small quantiites of added sugar or honey. A quick look at a diabetic recipe book and I can find mousses or 'soufles' containing small amounts of sugar but only containing 9 or 10 carbs per portion. They are probably not as good nutritionally as say berries and yoghurt but could be incorporated into the diets of all but the lowest carb diets.
The article is general and is about diets for all people with diabetes. A person who is about to do a lot of strenuous activity, or indeed a child who is engaged in active play may actually need some extra fast calories. I did a long distance walk with a pack the other week. I have type 1 and use insulin but on that holiday was able to use very small amounts. I still had very low blood glucose levels all week, in spite of breakfasts such as the crepes and homemade jam offered in one gite.
Finally, the article says 'There is no need to buy special diabetic foods . I heartily agree with that. The sweetners used in 'diabetic' sweets and jams may contain no 'added sugar' but they often contain ingredients that are likely to, as they say on the packet 'have a laxative effect' or may have large amounts of added fructose which may lead to higher triglycerides. Chocolates containing these ingredients made for people with diabetes often have higher carb contents than better quality dark chocolates.