Did you notice that I quoted directly from the question that you asked The Alabama Crimson Tide Abbey Road signatures shirt? The answer to your question appears in the sentence immediately prior to your question. In other words, you answered yourself before you asked anybody else. The reason it was hard and small was that the bakery left the baking powder out! Baking powder is a “Leavening agent” it works in 2 ways, it produces small carbon dioxide bubbles when it gets wet, and again when exposed to heat in the oven. Leave this out, and only “air” that got beaten in while mixing the cake has any chance of causing ANY rising. Eggs, and egg whites, in particular, beaten until they form the so-called ‘stiff peaks’ meringue also works. The secret here is to incorporate the flour as quickly as possible, yet without ‘beating’ the batter, just gently fold it in as not to lose the fluffiness you’ve worked so hard to achieve.
A fun way to check if you’ve beaten it into stiff peaks is to turn the bowl upside down. If the meringue doesn’t plop on the floor but stays in the bowl as if a solid, then you’ve succeeded. If you don’t have time to scrape it off and make the new frosting, add some berries to the cake, tartness will help balance out the flavor a bit. With sour fruits, you will welcome the sweetness. If it’s a chocolate cake or one that goes well with chocolate, sprinkle some cocoa powder on top. The bitterness may offset the sweetness as well. Bakeries are in business to make money, just like everybody else. And larger bakeries or grocery store bakeries tend to be more about the bottom line than quality, whereas small, independent bakeries tend to really focus on providing the best possible products because their reputations are their greatest assets. If freshness is important, you’re better off in a small bakery or special-ordering.