You aren't allowed to put things on the ingredients list which aren't actually in the food, and the new regulation doesn't let they say that there _might_ be sesame.
There's two conflicting laws here - one says you MUST label if you have allergens (this one allows the "may or may not contain the devil's seeds") and another says you cannot imply or state your product has something it does not have (this appears to be the one banning "may or may not").
"May contain x" is not part of the ingredients list, though. An ingredient is something intentionally in the product. The warning is just telling you that there's a chance that x accidentally got in.