Consider the following use case. We have Confluence Cloud (that was in Cloud from the beginning, it has not been migrated) and we have Jira DC. Confluence Cloud has various links to Jira DC. Now we migrate Jira DC to Cloud. This means we want to fix link in Confluence Cloud that are still pointing to Jira DC. We cannot do it via the UI, because Product links UI will not show Confluence Cloud in the products table as product eligible for fixing links, because it has never been migrated.

This is purely a UI limitation as the fixing links system is capable of fixing links in Confluence Cloud in such case. As a workaround one can use REST API (https://developer.atlassian.com/cloud/migration/rest/api-group-product-links/#api-jobs-cma-post) which is more flexible that UI in this regard.