I doubt the Legacy winglet stayed attached more than a foot or so; so, likely would not slice the entire span of lower wing skin. If it got that far it would likely have remained attached.
We are talking a 1000 kt impact which can do damage in a number of ways including:
- sever hydraulic and electric lines
- airloads pulling off wingskin and making controllability issues
- fuel tank rupture and ignition of contents at the moment of collision.