wpgraft needs no access to your Wix account, no export file and no API key. It reads the page as your browser rendered it: the real elements and the real computed styles on each one, at every width you ask for. It then builds that as a Shopify section made of theme blocks, with every style in one of the blocks’ own settings. Nothing that arrives depends on Wix still being there.
Wix has no way to export a design, and Shopify has no way to import one, so a store leaving Wix normally rebuilds every page in the theme editor by hand. wpgraft copies from the published Wix site and writes each page or section into your Shopify theme as theme blocks, with the styles in their settings. Products, orders and customers are a separate move; this is the design half of it.