Not particularly good advice as she is discussing the policies for her bakery and not the what every bakery does. In fact if you go to Carlo's Bakery's website Buddy states precisely the opposite (only using this example because I am familiar with the site).

Frankly his position makes much more sense, they charge a flat fee for their additional sheet cake. Her justification for the servings costing the same is nonsensical as the factored into the cost of the wedding cake would be the level of decoration, additional supplies in the forms of structural supports, non icing decorations such as flowers etc which would not be used to "decorate" back up sheet cake which no one sees.

Charging the same price per serving enables her to make the most she can off each wedding cake (good for her) but bad advice as not as all bakeries follow the same pricing and suggesting so would be disingenuous.

I wouldn't call it disingenuous just because it pertains to what she does...Everyone does things differently, I think that people are smart enough to know that. I charge almost the same for sheet cakes as for wedding cakes, based on the fact that it's extra baking time if someone goes from a three-tier that serves 100 to a three-tier that serve 75 plus a sheet cake. You just added time to my schedule in baking, filling and icing an extra cake. If she gives advice for her bakery more power to her, people are always free to shop around.