It was a simple mistake about confusing ssj2 with 1 against Cell, but it still doesn't change the fact that he can not win in a energy battle, because Thanos himself will not allow it to be just purely a long range battle without gaining an advantage, which he has many which was already explained.
Was it also a "simple mistake" when you decided a Kamehameha could never harm Thor because it wasn't "hot enough"?