wpgraft needs no access to your Squarespace 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 Squarespace still being there.
Squarespace exports content, not design, and nothing it exports opens in a Shopify theme. wpgraft takes the design from the published site, Fluid Engine grids included, and writes it into your Shopify theme as blocks you edit in the theme editor. Your products move separately; the pages they sit on can move with this.