“Besides, a difference in price on the same product is objective. Why wouldn’t it be l’toeles for everyone reading it?”
According to that logic, this halacha wouldn’t exist in the first place. Clearly it is not always permitted to tell someone something about merchandise even when it’s true. It is only permissible in certain circumstances. It would be impossible for all of those conditions to be fulfilled in this kind of setting since they would have to be fulfilled for each person reading it and what and how you are allowed to say something to one person would be different than to another.