The ability to hold a series of numbers in one's head while working a problem is different that being able to mentally assess and manipulate multiple pieces to a problem in one's head, and it isn't clear that both require the same frontal lobe access to the brain. Many dyslexics have frontal lobe issues that affect working memory but are excellent at "3-D" thinking. The 2E folks in my home are amazing at 3-D thinking but not at idling numbers in their head. That is partially why I think the concept of working memory determining giftedness isn't an absolute or accurate.

Helpful, yes. Necessary? No.