No where in the article does it say allergens were added. If they were then the labelling could not possibly "misbranded".
> Because it can be difficult and expensive to keep sesame in one part of a baking plant out of another, some companies began adding small amounts of sesame to products that didn't previously contain the ingredient to avoid liability and cost.
So yes, to avoid having to prevent cross-contamination, they started intentionally introducing trace amounts of allergens.
Produce an item, clean your utensils before the next time. Minimal cross-contamination issues.
A production line processing sesame will create a certain amount of sesame dust. Ensuring that dust gets nowhere near other production equipment is expensive.