As the Include Bitbucket for Confluence Data Center app authenticates against Bitbucket Cloud with a shared Username and App Password, the Confluence users have access to data from all repositories that the configured Cloud user has access to.
More information available about authentication for Bitbucket Cloud in this section.
Bitbucket Data Center
The Connection to Bitbucket Data Center instances is handled through App links. Each user’s individual permissions in the said instances are enforced in Confluence.
In other words, a user can only see data from repositories that he has access to in your Bitbucket instances.