sharepoint online – Is it a requirement to have the users synchronized and licensed for a Hybrid federated search?

I want to create a hybrid search scenario in which users can get results from both SharePoint 2013 server and SharePoint online on the SharePoint 2013 search , but NOT the other way around, do I need to synchronize all users accounts to 365 and assign them licenses or can I just do that for the account that used for the Search Service application on the local server?

The road map for hybrid federated search suggest as a first step :

Configure your Microsoft 365 organization for a hybrid environment, including registering your domain, configuring UPN suffixes, and synchronizing your user accounts.

But since hybrid federated search uses two indexes and I just want to display results from O365 and SharePoint 2013 on SharePoint 2013, I don’t understand why the users accounts performing the search on the local server would need to be synchronized to O365.