This export Designer pipeline to code feature is only available in 1P subscriptions. Currently not exposed this feature because the py code exported are based on a internal version of SDK which mainly support 1P customers . The code there will most likely be changed when we preview SDK 2.0 with 3P customer.
At the meanwhile, we heard a lot feedbacks about the importance for export/import feature as 3P customers want to leverage this feature to move their pipeline from 1 workspace to another or archive their pipeline to code to integrate with Github etc. most likely will private preview the “export pipeline to Yaml” with DevPlat V2 private preview.