That's the one thing about the digestive biscuit that doesn't fit...they are not really very sweet. Sweet, but not really sweet, like I imagine a honey-cake would be.
But then I think that during the time Tolkien was writing, there might have been restrictions on sugar or something...or peoples idea of sweet was different. For example, I know in Mexico, their cookies are a lot less sweet than I am used to.
So perhaps they seemed very sweet, at least to Tolkien, at the time.
The honeycakes of the Beornings has always reminded me of Baklava!