I'm sorry about this, but I'm a veggie gardener as well as a reader (and cook) of all sorts of recipes!
Buckwheat is plant, which flowers in the summer and is sensitive to frost. (The grain is the seed which follows the flower.) Hence the description "summer annual".
But does the sentence have any sense ''it sounds like grain but it only grows in the summer'? I can't make any sense of this. I found that buckwheat is not grain but a kind of vegetable, so it could be that, but... Nevermind, thank you P.
"Buckwheat is often mistakenly described as a grain. In fact, buckwheat "grain " is actually a fruit seed. As it contains no gluten it is particularly suitable for people who have wheat or gluten intolerance." (or from the point of view of the thingy diet, people who wish to avoid wheat products)