There are some things wrong with your line of thinking.

First of all never mix up your personal headcanon with facts.

I don't know who ever claimed Flutterbat to be the same as bat ponies (aka Lunar Guard/thestrals)

The reason why it's a commonly accepted fact of the community is real life logic. Most bats eat fruit, some exclusively. Ponies and horses have quite the sweet tooth and will prefer sweet fruits over hay or grass.

So batponies will go absolutely nuts over fruits.

Also it's just super adorable.

If thestrals don't like fruit in your headcanon then that's fine for you and you are welcome to argue why it would be beneficial for artists or fanfic writers to depict them as such.

But calling something that an extreme majority of the fandom agrees on a misconception because it doesn't fit into your headcanon just makes no sense.