Yes, I can confirm that using the GraphQL Admin API is the supported and ideal method of getting sorted results from Shopify.
From my knowledge, the default sort order of the orders.json REST endpoint is the “id” field in ascending order. The “id” is generated incrementally every time an order occurs on Shopify, so you can also intepret this sort order and ordered by the “created_at” date time in ascending order (i.e. older order first).