I believe that it's not possible to sync existing QBO objects to HubSpot products as this Quickbooks help documentation states: Please note: only newly created products will sync to HubSpot."
The reason for this is that the integration API relies on a trigger to sync. Existing products will be added to HubSpot if they are updated within QuickBooks. HubSpot relies on the creation of or updating products to sync them from Quickbooks.
A potential workaround for this is to perform a bulk update by exporting and then reimporting your products all within Quickbooks.
This is indeed a limitation of the QuickBooks Online integration: it only allows for new items to sync to HubSpot but not old ones. The reason for this is that the API relies on a trigger to sync, existing products will be added only if they are updated within QuickBooks.