Happy to clarify.
The Ecommerce Bridge allows you to create, update, and delete contacts, deals, products, and line items without having to use those dedicated APIs.
For example, to create a contact once you've installed app or the ecommerce settings and configured your property mappings, you could send a sync message like the example below:
You could also create a deal with a sync message like:
This is all done through the Ecommerce Bridge; you don't have to use the Contacts API and Deals API separately.
Are you developing a system for only your organization to use, or do you want or need to share it with other HubSpot users? If the system is only for internal use, you can develop a custom integration. If you plan to develop an app that can be used by others, then you will need to implement an OAuth Ecommerce Bridge application. If you are implementing an app, then yes, the import process syncs current data.
I should note that are existing Ecommerce Bridge apps that support Magento. You may be interested in using the following: