The premise of Myne selling the pound cake recipe to Frieda and her chef is that sugar is new and nobody really knows how to use it. If we compare this to our own history, is it believable that nobody would know how to bake confections? Humans are pretty resourceful at figuring out what to do with stuff. Though can take time and luck, like with how we figured out we can get yeast and use it to make bread.
From my research, the answer seems to be yes and no.
Sugar existed in other places at earlier points, but medieval Europe most closely matches the fantasy world Myne lives in, and a case could be made that Europeans didn't know what to with sugar for a while. In fact, just like in Myne's world, medieval Europe had sugar introduced to them from outside (from the Arab world), and it was quite expensive, like many other spices (see my answer here for some info on the spice trade). And like in ancient Greece, sugar was at first mainly viewed as medicine.
According to this site that has some primary sources, sugar would have come to Europe from the Arab world during the span between the 8th and 12th centuries. In the late 11th century, we get one of the earliest mentions of sugar in England, in a medical text.
So far, this is looking good for Ascendance of a Bookworm. When sugar was first introduced in Europe, people thought of it as more of a medicine than something to bake with. The case for the pound cake in particular is good. Though Myne says it's a pretty simple recipe, most sources I'm looking at say it was invented sometime in the 1700s in Britain, with it definitely appearing by 1796 in America, as evidenced by the cookbook American Cookery: or, The Art of Dressing Viands, Fish, Poultry and Vegetables, and the Best Modes of Making Puff-pastes, Pies, Tarts, Puddings, Custards and Preserves, and all kinds of Cakes, from the Imperial Plumb to plain Cake by Amelia Simmons (https://whatscookingamerica.net/history/cakes/poundcake.htm)
So by inventing pound cake, Myne did move the timeline up by some amount of centuries.
Now it's time for some problems to the narrative we should consider. First, the creation of sweets in general goes back very far, all the way to the ancient Egyptians, who used honey combined with fruits and nuts to make candy. And there are actually medieval recipe books available that show honey was also used in the medieval ages to make sweets. This includes this German cook book from 1553 and the English The Forme of Cury in 1390. The stuff the English made included cakes, custards, fritters, and cheesecake, and the Germans had recipes for tarts, pudding, and pastries (http://www.eg.bucknell.edu/~lwittie/sca/food/dessert.html). The writer on this bucknell site even notes this of the Germans.
One cookbook had a large section on how to sugarcoat virtually anything sweet. I've seen almost hundreds more sweet recipes in these few German cook books than in all the English cookbooks mentioned above. This suggests that either the Germans were very fond of sweets or the English didn't write down their recipes.
And the preface of The Forme of Cury actually notes that sugar was starting to be used in place of honey!
Sugar, or Sugur , was now beginning here to take place of honey; however, they are used together, No. 67. Sugar came from the Indies, by way of Damascus and Aleppo, to Venice, Genoa, and Pisa, and from these last places to us . It is here not only frequently used, but was of various sorts, as cypre, No. 41. 99. 120. named probably from the isle of Cyprus, whence it might either come directly to us, or where it had received some improvement by way of refining. There is mention of blanch-powder or white sugar, 132. They, however, were not the same, for see No. 193. Sugar was clarified sometimes with wine .
From these we can see that there were sweets similar to the ones Myne introduced around, and even that these cultures had an inclination to go down the route of sweets. At least as far as the books I mentioned though, they did come a bit later, so Myne's sweets as an innovation might still be safe.
One of the problems in Ascendance of a Bookworm is it's portrayal of Myne as sole innovator in cooking (and other things). Myne mentions a bunch of times how bland the food is, and nobody seems to be doing anything about it or trying to come up with new recipes. In particular, the widespread practice of parboiling vegetables and then throwing out the water is very wasteful in her eyes, and yet nobody but Myne is able to figure out that water that can be used for soup. This can be similar to Ancient Alien Theory suggesting ancient civilizations couldn't possible have been smart or innovative enough to come up with stuff on their own, and instead needed an outside source. This is obviously incorrect and disrespectful.
However, I don't believe we need to overly worry about Ascendance of a Bookworm with this regard, as it does subvert it some ways. Though the original ideas may come from Myne, Leise improve on the pound cake recipe by experimenting on her own. And in the framework of this story, Myne truly does exist as an outside source bringing in new ideas. It truthfully has to be said that Myne's modern knowledge would be a boon to this fantasy society that seems at least a few hundred years behind our own.
I really enjoy this series, and while I haven't done enough research to either refute or prove the idea entirely, I think the historic information I've put forth shows that there is a framework in which the suspension of disbelief is kept intact with regards to Myne's development of sweets.