That's total horse horseshit. The fact that Sting has never be in the WWE has absolutely no effect on his legend status. Who the hell would think any less of him?
I would definitely think less of him if he concludes his career without having competed in WWE, even at his advanced age. I'm not saying the guy isn't a legend, but the extent of his legacy is definitely minimized without at least a brief appearance on the grandest stage of them all. It's a Doug Flutie analogy.