Cooking potatoes - baking covered in salt for mashing
The only answer that makes any sense (other than the "just for show" hypothesis) is that baking in salt does create a fluffier potato. That assertion is backed up by Cook's Illustrated and the Idaho Potato Commission [citation]. So, for the lightest possible gnocchi, start with the fluffiest possible potato.

I've got to say though, roasting potatoes in salt for gnocchi would be too fussy even for me. 

EDIT: One other possibility just occurred to me, and the more I think of it, the more I think it's the key. Maybe the restaurant always bakes potatoes that way and it's just as easy to add extra for the gnocchi or to use leftovers. So, they wrote the recipe the way they actually do it.